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       The functions in this section can serve as terms in an
       expression.  They fall into two major categories: list
       operators and named unary operators.  These differ in
       their precedence relationship with a following comma.
       (See the precedence table in perlop.)  List operators take
       more than one argument, while unary operators can never
       take more than one argument.  Thus, a comma terminates the
       argument of a unary operator, but merely separates the
       arguments of a list operator.  A unary operator generally
       provides a scalar context to its argument, while a list
       operator may provide either scalar or list contexts for
       its arguments.  If it does both, the scalar arguments will
       be first, and the list argument will follow.  (Note that
       there can ever be only one such list argument.)  For
       instance, splice() has three scalar arguments followed by
       a list, whereas gethostbyname() has four scalar arguments.

       In the syntax descriptions that follow, list operators
       that expect a list (and provide list context for the ele­
       ments of the list) are shown with LIST as an argument.
       Such a list may consist of any combination of scalar argu­
       ments or list values; the list values will be included in
       the list as if each individual element were interpolated
       at that point in the list, forming a longer single-dimen­
       sional list value.  Elements of the LIST should be sepa­
       rated by commas.

       Any function in the list below may be used either with or
       without parentheses around its arguments.  (The syntax
       descriptions omit the parentheses.)  If you use the paren­
       theses, the simple (but occasionally surprising) rule is
       this: It looks like a function, therefore it is a func­
       tion, and precedence doesn't matter.  Otherwise it's a
       list operator or unary operator, and precedence does mat­
       ter.  And whitespace between the function and left paren­
       thesis doesn't count--so you need to be careful sometimes:

           print 1+2+4;        # Prints 7.
           print(1+2) + 4;     # Prints 3.
           print (1+2)+4;      # Also prints 3!
           print +(1+2)+4;     # Prints 7.
           print ((1+2)+4);    # Prints 7.

       If you run Perl with the -w switch it can warn you about
       this.  For example, the third line above produces:

           print (...) interpreted as function at - line 1.
           Useless use of integer addition in void context at - line 1.

       A few functions take no arguments at all, and therefore
       work as neither unary nor list operators.  These include
       return the length of the list that would have been
       returned in list context.  Some operators return the first
       value in the list.  Some operators return the last value
       in the list.  Some operators return a count of successful
       operations.  In general, they do what you want, unless you
       want consistency.

       A named array in scalar context is quite different from
       what would at first glance appear to be a list in scalar
       context.  You can't get a list like "(1,2,3)" into being
       in scalar context, because the compiler knows the context
       at compile time.  It would generate the scalar comma oper­
       ator there, not the list construction version of the
       comma.  That means it was never a list to start with.

       In general, functions in Perl that serve as wrappers for
       system calls of the same name (like chown(2), fork(2),
       closedir(2), etc.) all return true when they succeed and
       "undef" otherwise, as is usually mentioned in the descrip­
       tions below.  This is different from the C interfaces,
       which return "-1" on failure.  Exceptions to this rule are
       "wait", "waitpid", and "syscall".  System calls also set
       the special $!  variable on failure.  Other functions do
       not, except accidentally.

       Perl Functions by Category

       Here are Perl's functions (including things that look like
       functions, like some keywords and named operators)
       arranged by category.  Some functions appear in more than
       one place.

       Functions for SCALARs or strings
           "chomp", "chop", "chr", "crypt", "hex", "index", "lc",
           "lcfirst", "length", "oct", "ord", "pack",
           "q/STRING/", "qq/STRING/", "reverse", "rindex",
           "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"

       Regular expressions and pattern matching
           "m//", "pos", "quotemeta", "s///", "split", "study",

       Numeric functions
           "abs", "atan2", "cos", "exp", "hex", "int", "log",
           "oct", "rand", "sin", "sqrt", "srand"

       Functions for real @ARRAYs
           "pop", "push", "shift", "splice", "unshift"

       Functions for list data
           "grep", "join", "map", "qw/STRING/", "reverse",
           "sort", "unpack"
           "unpack", "vec"

       Functions for filehandles, files, or directories
           "-X", "chdir", "chmod", "chown", "chroot", "fcntl",
           "glob", "ioctl", "link", "lstat", "mkdir", "open",
           "opendir", "readlink", "rename", "rmdir", "stat",
           "symlink", "sysopen", "umask", "unlink", "utime"

       Keywords related to the control flow of your perl program
           "caller", "continue", "die", "do", "dump", "eval",
           "exit", "goto", "last", "next", "redo", "return",
           "sub", "wantarray"

       Keywords related to scoping
           "caller", "import", "local", "my", "our", "package",

       Miscellaneous functions
           "defined", "dump", "eval", "formline", "local", "my",
           "our", "reset", "scalar", "undef", "wantarray"

       Functions for processes and process groups
           "alarm", "exec", "fork", "getpgrp", "getppid", "get­
           priority", "kill", "pipe", "qx/STRING/", "setpgrp",
           "setpriority", "sleep", "system", "times", "wait",

       Keywords related to perl modules
           "do", "import", "no", "package", "require", "use"

       Keywords related to classes and object-orientedness
           "bless", "dbmclose", "dbmopen", "package", "ref",
           "tie", "tied", "untie", "use"

       Low-level socket functions
           "accept", "bind", "connect", "getpeername", "getsock­
           name", "getsockopt", "listen", "recv", "send", "set­
           sockopt", "shutdown", "socket", "socketpair"

       System V interprocess communication functions
           "msgctl", "msgget", "msgrcv", "msgsnd", "semctl",
           "semget", "semop", "shmctl", "shmget", "shmread",

       Fetching user and group info
           "endgrent", "endhostent", "endnetent", "endpwent",
           "getgrent", "getgrgid", "getgrnam", "getlogin", "getp­
           went", "getpwnam", "getpwuid", "setgrent", "setpwent"

       Fetching network info
           "endprotoent", "endservent", "gethostbyaddr", "geth­
           ostbyname", "gethostent", "getnetbyaddr", "getnetby­

           * - "sub" was a keyword in perl4, but in perl5 it is
           an operator, which can be used in expressions.

       Functions obsoleted in perl5
           "dbmclose", "dbmopen"


       Perl was born in Unix and can therefore access all common
       Unix system calls.  In non-Unix environments, the func­
       tionality of some Unix system calls may not be available,
       or details of the available functionality may differ
       slightly.  The Perl functions affected by this are:

       "-X", "binmode", "chmod", "chown", "chroot", "crypt",
       "dbmclose", "dbmopen", "dump", "endgrent", "endhostent",
       "endnetent", "endprotoent", "endpwent", "endservent",
       "exec", "fcntl", "flock", "fork", "getgrent", "getgrgid",
       "gethostbyname", "gethostent", "getlogin", "getnetbyaddr",
       "getnetbyname", "getnetent", "getppid", "getprgp", "get­
       priority", "getprotobynumber", "getprotoent", "getpwent",
       "getpwnam", "getpwuid", "getservbyport", "getservent",
       "getsockopt", "glob", "ioctl", "kill", "link", "lstat",
       "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe",
       "readlink", "rename", "select", "semctl", "semget",
       "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
       "setpriority", "setprotoent", "setpwent", "setservent",
       "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
       "socket", "socketpair", "stat", "symlink", "syscall",
       "sysopen", "system", "times", "truncate", "umask",
       "unlink", "utime", "wait", "waitpid"

       For more information about the portability of these func­
       tions, see perlport and other available platform-specific

       Alphabetical Listing of Perl Functions

       -X EXPR
       -X      A file test, where X is one of the letters listed
               below.  This unary operator takes one argument,
               either a filename or a filehandle, and tests the
               associated file to see if something is true about
               it.  If the argument is omitted, tests $_, except
               for "-t", which tests STDIN.  Unless otherwise
               documented, it returns 1 for true and '' for
               false, or the undefined value if the file doesn't
               exist.  Despite the funny names, precedence is the
               same as any other named unary operator, and the
                   -z  File has zero size (is empty).
                   -s  File has nonzero size (returns size in bytes).

                   -f  File is a plain file.
                   -d  File is a directory.
                   -l  File is a symbolic link.
                   -p  File is a named pipe (FIFO), or Filehandle is a pipe.
                   -S  File is a socket.
                   -b  File is a block special file.
                   -c  File is a character special file.
                   -t  Filehandle is opened to a tty.

                   -u  File has setuid bit set.
                   -g  File has setgid bit set.
                   -k  File has sticky bit set.

                   -T  File is an ASCII text file (heuristic guess).
                   -B  File is a "binary" file (opposite of -T).

                   -M  Script start time minus file modification time, in days.
                   -A  Same for access time.
                   -C  Same for inode change time (Unix, may differ for other platforms)


                   while (<>) {
                       next unless -f $_;      # ignore specials

               The interpretation of the file permission opera­
               tors "-r", "-R", "-w", "-W", "-x", and "-X" is by
               default based solely on the mode of the file and
               the uids and gids of the user.  There may be other
               reasons you can't actually read, write, or execute
               the file.  Such reasons may be for example network
               filesystem access controls, ACLs (access control
               lists), read-only filesystems, and unrecognized
               executable formats.

               Also note that, for the superuser on the local
               filesystems, the "-r", "-R", "-w", and "-W" tests
               always return 1, and "-x" and "-X" return 1 if any
               execute bit is set in the mode.  Scripts run by
               the superuser may thus need to do a stat() to
               determine the actual mode of the file, or tem­
               porarily set their effective uid to something

               If you are using ACLs, there is a pragma called
               "filetest" that may produce more accurate results
               expected, however--only single letters following a
               minus are interpreted as file tests.

               The "-T" and "-B" switches work as follows.  The
               first block or so of the file is examined for odd
               characters such as strange control codes or char­
               acters with the high bit set.  If too many strange
               characters (>30%) are found, it's a "-B" file,
               otherwise it's a "-T" file.  Also, any file con­
               taining null in the first block is considered a
               binary file.  If "-T" or "-B" is used on a file­
               handle, the current IO buffer is examined rather
               than the first block.  Both "-T" and "-B" return
               true on a null file, or a file at EOF when testing
               a filehandle.  Because you have to read a file to
               do the "-T" test, on most occasions you want to
               use a "-f" against the file first, as in "next
               unless -f $file && -T $file".

               If any of the file tests (or either the "stat" or
               "lstat" operators) are given the special filehan­
               dle consisting of a solitary underline, then the
               stat structure of the previous file test (or stat
               operator) is used, saving a system call.  (This
               doesn't work with "-t", and you need to remember
               that lstat() and "-l" will leave values in the
               stat structure for the symbolic link, not the real
               file.)  (Also, if the stat buffer was filled by a
               "lstat" call, "-T" and "-B" will reset it with the
               results of "stat _").  Example:

                   print "Can do.\n" if -r $a || -w _ || -x _;

                   print "Readable\n" if -r _;
                   print "Writable\n" if -w _;
                   print "Executable\n" if -x _;
                   print "Setuid\n" if -u _;
                   print "Setgid\n" if -g _;
                   print "Sticky\n" if -k _;
                   print "Text\n" if -T _;
                   print "Binary\n" if -B _;

       abs VALUE
       abs     Returns the absolute value of its argument.  If
               VALUE is omitted, uses $_.

               Accepts an incoming socket connect, just as the
               accept(2) system call does.  Returns the packed
               address if it succeeded, false otherwise.  See the
               example in "Sockets: Client/Server Communication"
               how seconds are counted, and process scheduling
               may delay the delivery of the signal even fur­

               Only one timer may be counting at once.  Each call
               disables the previous timer, and an argument of 0
               may be supplied to cancel the previous timer with­
               out starting a new one.  The returned value is the
               amount of time remaining on the previous timer.

               For delays of finer granularity than one second,
               you may use Perl's four-argument version of
               select() leaving the first three arguments unde­
               fined, or you might be able to use the "syscall"
               interface to access setitimer(2) if your system
               supports it.  The Time::HiRes module (from CPAN,
               and starting from Perl 5.8 part of the standard
               distribution) may also prove useful.

               It is usually a mistake to intermix "alarm" and
               "sleep" calls.  ("sleep" may be internally imple­
               mented in your system with "alarm")

               If you want to use "alarm" to time out a system
               call you need to use an "eval"/"die" pair.  You
               can't rely on the alarm causing the system call to
               fail with $! set to "EINTR" because Perl sets up
               signal handlers to restart system calls on some
               systems.  Using "eval"/"die" always works, modulo
               the caveats given in "Signals" in perlipc.

                   eval {
                       local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
                       alarm $timeout;
                       $nread = sysread SOCKET, $buffer, $size;
                       alarm 0;
                   if ($@) {
                       die unless $@ eq "alarm\n";   # propagate unexpected errors
                       # timed out
                   else {
                       # didn't

               For more information see perlipc.

       atan2 Y,X
               Returns the arctangent of Y/X in the range -PI to

               For the tangent operation, you may use the

       binmode FILEHANDLE
               Arranges for FILEHANDLE to be read or written in
               "binary" or "text" mode on systems where the run-
               time libraries distinguish between binary and text
               files.  If FILEHANDLE is an expression, the value
               is taken as the name of the filehandle.  Returns
               true on success, otherwise it returns "undef" and
               sets $! (errno).

               On some systems (in general, DOS and Windows-based
               systems) binmode() is necessary when you're not
               working with a text file.  For the sake of porta­
               bility it is a good idea to always use it when
               appropriate, and to never use it when it isn't
               appropriate.  Also, people can set their I/O to be
               by default UTF-8 encoded Unicode, not bytes.

               In other words: regardless of platform, use bin­
               mode() on binary data, like for example images.

               If LAYER is present it is a single string, but may
               contain multiple directives. The directives alter
               the behaviour of the file handle.  When LAYER is
               present using binmode on text file makes sense.

               If LAYER is omitted or specified as ":raw" the
               filehandle is made suitable for passing binary
               data. This includes turning off possible CRLF
               translation and marking it as bytes (as opposed to
               Unicode characters).  Note that as despite what
               may be implied in "Programming Perl" (the Camel)
               or elsewhere ":raw" is not the simply inverse of
               ":crlf" -- other layers which would affect binary
               nature of the stream are also disabled. See Per­
               lIO, perlrun and the discussion about the PERLIO
               environment variable.

               The ":bytes", ":crlf", and ":utf8", and any other
               directives of the form ":...", are called I/O lay­
               ers.  The "open" pragma can be used to establish
               default I/O layers.  See open.

               The LAYER parameter of the binmode() function is
               described as "DISCIPLINE" in "Programming Perl,
               3rd Edition".  However, since the publishing of
               this book, by many known as "Camel III", the con­
               sensus of the naming of this functionality has
               moved from "discipline" to "layer".  All documen­
               tation of this version of Perl therefore refers to
               "layers" rather than to "disciplines".  Now back
               to the regularly scheduled documentation...

               The operating system, device drivers, C libraries,
               and Perl run-time system all work together to let
               the programmer treat a single character ("\n") as
               the line terminator, irrespective of the external
               representation.  On many operating systems, the
               native text file representation matches the inter­
               nal representation, but on some platforms the
               external representation of "\n" is made up of more
               than one character.

               Mac OS, all variants of Unix, and Stream_LF files
               on VMS use a single character to end each line in
               the external representation of text (even though
               that single character is CARRIAGE RETURN on Mac OS
               and LINE FEED on Unix and most VMS files). In
               other systems like OS/2, DOS and the various fla­
               vors of MS-Windows your program sees a "\n" as a
               simple "\cJ", but what's stored in text files are
               the two characters "\cM\cJ".  That means that, if
               you don't use binmode() on these systems, "\cM\cJ"
               sequences on disk will be converted to "\n" on
               input, and any "\n" in your program will be con­
               verted back to "\cM\cJ" on output.  This is what
               you want for text files, but it can be disastrous
               for binary files.

               Another consequence of using binmode() (on some
               systems) is that special end-of-file markers will
               be seen as part of the data stream.  For systems
               from the Microsoft family this means that if your
               binary data contains "\cZ", the I/O subsystem will
               regard it as the end of the file, unless you use

               binmode() is not only important for readline() and
               print() operations, but also when using read(),
               seek(), sysread(), syswrite() and tell() (see
               perlport for more details).  See the $/ and "$\"
               variables in perlvar for how to manually set your
               input and output line-termination sequences.

       bless REF,CLASSNAME
       bless REF
               This function tells the thingy referenced by REF
               that it is now an object in the CLASSNAME package.
               If CLASSNAME is omitted, the current package is
               used.  Because a "bless" is often the last thing
               in a constructor, it returns the reference for
               convenience.  Always use the two-argument version
               if the function doing the blessing might be inher­
               ited by a derived class.  See perltoot and perlobj

       caller  Returns the context of the current subroutine
               call.  In scalar context, returns the caller's
               package name if there is a caller, that is, if
               we're in a subroutine or "eval" or "require", and
               the undefined value otherwise.  In list context,

                   ($package, $filename, $line) = caller;

               With EXPR, it returns some extra information that
               the debugger uses to print a stack trace.  The
               value of EXPR indicates how many call frames to go
               back before the current one.

                   ($package, $filename, $line, $subroutine, $hasargs,
                   $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);

               Here $subroutine may be "(eval)" if the frame is
               not a subroutine call, but an "eval".  In such a
               case additional elements $evaltext and $is_require
               are set: $is_require is true if the frame is cre­
               ated by a "require" or "use" statement, $evaltext
               contains the text of the "eval EXPR" statement.
               In particular, for an "eval BLOCK" statement,
               $filename is "(eval)", but $evaltext is undefined.
               (Note also that each "use" statement creates a
               "require" frame inside an "eval EXPR" frame.)
               $subroutine may also be "(unknown)" if this par­
               ticular subroutine happens to have been deleted
               from the symbol table.  $hasargs is true if a new
               instance of @_ was set up for the frame.  $hints
               and $bitmask contain pragmatic hints that the
               caller was compiled with.  The $hints and $bitmask
               values are subject to change between versions of
               Perl, and are not meant for external use.

               Furthermore, when called from within the DB pack­
               age, caller returns more detailed information: it
               sets the list variable @DB::args to be the argu­
               ments with which the subroutine was invoked.

               Be aware that the optimizer might have optimized
               call frames away before "caller" had a chance to
               get the information.  That means that caller(N)
               might not return information about the call frame
               you expect it do, for "N > 1".  In particular,
               @DB::args might have information from the previous
               time "caller" was called.

       chdir EXPR
               Changes the working directory to EXPR, if possi­
               ble. If EXPR is omitted, changes to the directory
               "oct", if all you have is a string.

                   $cnt = chmod 0755, 'foo', 'bar';
                   chmod 0755, @executables;
                   $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to
                                                            # --w----r-T
                   $mode = '0644'; chmod oct($mode), 'foo'; # this is better
                   $mode = 0644;   chmod $mode, 'foo';      # this is best

               You can also import the symbolic "S_I*" constants
               from the Fcntl module:

                   use Fcntl ':mode';

                   chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
                   # This is identical to the chmod 0755 of the above example.

       chomp VARIABLE
       chomp( LIST )
       chomp   This safer version of "chop" removes any trailing
               string that corresponds to the current value of $/
               (also known as $INPUT_RECORD_SEPARATOR in the
               "English" module).  It returns the total number of
               characters removed from all its arguments.  It's
               often used to remove the newline from the end of
               an input record when you're worried that the final
               record may be missing its newline.  When in para­
               graph mode ("$/ = """), it removes all trailing
               newlines from the string.  When in slurp mode ("$/
               = undef") or fixed-length record mode ($/ is a
               reference to an integer or the like, see perlvar)
               chomp() won't remove anything.  If VARIABLE is
               omitted, it chomps $_.  Example:

                   while (<>) {
                       chomp;  # avoid \n on last field
                       @array = split(/:/);
                       # ...

               If VARIABLE is a hash, it chomps the hash's val­
               ues, but not its keys.

               You can actually chomp anything that's an lvalue,
               including an assignment:

                   chomp($cwd = `pwd`);
                   chomp($answer = <STDIN>);

               If you chomp a list, each element is chomped, and
               the total number of characters removed is
               nor copies the string.  If VARIABLE is omitted,
               chops $_.  If VARIABLE is a hash, it chops the
               hash's values, but not its keys.

               You can actually chop anything that's an lvalue,
               including an assignment.

               If you chop a list, each element is chopped.  Only
               the value of the last "chop" is returned.

               Note that "chop" returns the last character.  To
               return all but the last character, use "sub­
               str($string, 0, -1)".

               See also "chomp".

       chown LIST
               Changes the owner (and group) of a list of files.
               The first two elements of the list must be the
               numeric uid and gid, in that order.  A value of -1
               in either position is interpreted by most systems
               to leave that value unchanged.  Returns the number
               of files successfully changed.

                   $cnt = chown $uid, $gid, 'foo', 'bar';
                   chown $uid, $gid, @filenames;

               Here's an example that looks up nonnumeric uids in
               the passwd file:

                   print "User: ";
                   chomp($user = <STDIN>);
                   print "Files: ";
                   chomp($pattern = <STDIN>);

                   ($login,$pass,$uid,$gid) = getpwnam($user)
                       or die "$user not in passwd file";

                   @ary = glob($pattern);      # expand filenames
                   chown $uid, $gid, @ary;

               On most systems, you are not allowed to change the
               ownership of the file unless you're the superuser,
               although you should be able to change the group to
               any of your secondary groups.  On insecure sys­
               tems, these restrictions may be relaxed, but this
               is not a portable assumption.  On POSIX systems,
               you can detect this condition this way:

                   use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
                   $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

               masked to the low eight bits.

               See perlunicode and encoding for more about Uni­

       chroot FILENAME
       chroot  This function works like the system call by the
               same name: it makes the named directory the new
               root directory for all further pathnames that
               begin with a "/" by your process and all its chil­
               dren.  (It doesn't change your current working
               directory, which is unaffected.)  For security
               reasons, this call is restricted to the superuser.
               If FILENAME is omitted, does a "chroot" to $_.

       close FILEHANDLE
       close   Closes the file or pipe associated with the file
               handle, returning true only if IO buffers are suc­
               cessfully flushed and closes the system file
               descriptor.  Closes the currently selected file­
               handle if the argument is omitted.

               You don't have to close FILEHANDLE if you are
               immediately going to do another "open" on it,
               because "open" will close it for you.  (See
               "open".)  However, an explicit "close" on an input
               file resets the line counter ($.), while the
               implicit close done by "open" does not.

               If the file handle came from a piped open "close"
               will additionally return false if one of the other
               system calls involved fails or if the program
               exits with non-zero status.  (If the only problem
               was that the program exited non-zero $! will be
               set to 0.)  Closing a pipe also waits for the pro­
               cess executing on the pipe to complete, in case
               you want to look at the output of the pipe after­
               wards, and implicitly puts the exit status value
               of that command into $?.

               Prematurely closing the read end of a pipe (i.e.
               before the process writing to it at the other end
               has closed it) will result in a SIGPIPE being
               delivered to the writer.  If the other end can't
               handle that, be sure to read all the data before
               closing the pipe.


                   open(OUTPUT, '|sort >foo')  # pipe to sort
                       or die "Can't start sort: $!";
                   #...                        # print stuff to output

       connect SOCKET,NAME
               Attempts to connect to a remote socket, just as
               the connect system call does.  Returns true if it
               succeeded, false otherwise.  NAME should be a
               packed address of the appropriate type for the
               socket.  See the examples in "Sockets:
               Client/Server Communication" in perlipc.

       continue BLOCK
               Actually a flow control statement rather than a
               function.  If there is a "continue" BLOCK attached
               to a BLOCK (typically in a "while" or "foreach"),
               it is always executed just before the conditional
               is about to be evaluated again, just like the
               third part of a "for" loop in C.  Thus it can be
               used to increment a loop variable, even when the
               loop has been continued via the "next" statement
               (which is similar to the C "continue" statement).

               "last", "next", or "redo" may appear within a
               "continue" block.  "last" and "redo" will behave
               as if they had been executed within the main
               block.  So will "next", but since it will execute
               a "continue" block, it may be more entertaining.

                   while (EXPR) {
                       ### redo always comes here
                   } continue {
                       ### next always comes here
                       # then back the top to re-check EXPR
                   ### last always comes here

               Omitting the "continue" section is semantically
               equivalent to using an empty one, logically
               enough.  In that case, "next" goes directly back
               to check the condition at the top of the loop.

       cos EXPR
       cos     Returns the cosine of EXPR (expressed in radians).
               If EXPR is omitted, takes cosine of $_.

               For the inverse cosine operation, you may use the
               "Math::Trig::acos()" function, or use this rela­

                   sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

       crypt PLAINTEXT,SALT
               Encrypts a string exactly like the crypt(3) func­

               When verifying an existing encrypted string you
               should use the encrypted text as the salt (like
               "crypt($plain, $crypted) eq $crypted").  This
               allows your code to work with the standard crypt
               and with more exotic implementations.  In other
               words, do not assume anything about the returned
               string itself, or how many bytes in the encrypted
               string matter.

               Traditionally the result is a string of 13 bytes:
               two first bytes of the salt, followed by 11 bytes
               from the set "[./0-9A-Za-z]", and only the first
               eight bytes of the encrypted string mattered, but
               alternative hashing schemes (like MD5), higher
               level security schemes (like C2), and implementa­
               tions on non-UNIX platforms may produce different

               When choosing a new salt create a random two char­
               acter string whose characters come from the set
               "[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9,
               'A'..'Z', 'a'..'z')[rand 64, rand 64]").  This set
               of characters is just a recommendation; the char­
               acters allowed in the salt depend solely on your
               system's crypt library, and Perl can't restrict
               what salts "crypt()" accepts.

               Here's an example that makes sure that whoever
               runs this program knows their own password:

                   $pwd = (getpwuid($<))[1];

                   system "stty -echo";
                   print "Password: ";
                   chomp($word = <STDIN>);
                   print "\n";
                   system "stty echo";

                   if (crypt($word, $pwd) ne $pwd) {
                       die "Sorry...\n";
                   } else {
                       print "ok\n";

               Of course, typing in your own password to whoever
               asks you for it is unwise.

               The crypt function is unsuitable for encrypting
               large quantities of data, not least of all because
               you can't get the information back.  Look at the
               by-module/Crypt and by-module/PGP directories on

               Breaks the binding between a DBM file and a hash.

       dbmopen HASH,DBNAME,MASK
               [This function has been largely superseded by the
               "tie" function.]

               This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or
               Berkeley DB file to a hash.  HASH is the name of
               the hash.  (Unlike normal "open", the first argu­
               ment is not a filehandle, even though it looks
               like one).  DBNAME is the name of the database
               (without the .dir or .pag extension if any).  If
               the database does not exist, it is created with
               protection specified by MASK (as modified by the
               "umask").  If your system supports only the older
               DBM functions, you may perform only one "dbmopen"
               in your program.  In older versions of Perl, if
               your system had neither DBM nor ndbm, calling
               "dbmopen" produced a fatal error; it now falls
               back to sdbm(3).

               If you don't have write access to the DBM file,
               you can only read hash variables, not set them.
               If you want to test whether you can write, either
               use file tests or try setting a dummy hash entry
               inside an "eval", which will trap the error.

               Note that functions such as "keys" and "values"
               may return huge lists when used on large DBM
               files.  You may prefer to use the "each" function
               to iterate over large DBM files.  Example:

                   # print out history file offsets
                   while (($key,$val) = each %HIST) {
                       print $key, ' = ', unpack('L',$val), "\n";

               See also AnyDBM_File for a more general descrip­
               tion of the pros and cons of the various dbm
               approaches, as well as DB_File for a particularly
               rich implementation.

               You can control which DBM library you use by load­
               ing that library before you call dbmopen():

                   use DB_File;
                   dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
                       or die "Can't open netscape history file: $!";

               tion: "pop" returns "undef" when its argument is
               an empty array, or when the element to return hap­
               pens to be "undef".

               You may also use "defined(&func)" to check whether
               subroutine &func has ever been defined.  The
               return value is unaffected by any forward declara­
               tions of &func.  Note that a subroutine which is
               not defined may still be callable: its package may
               have an "AUTOLOAD" method that makes it spring
               into existence the first time that it is called --
               see perlsub.

               Use of "defined" on aggregates (hashes and arrays)
               is deprecated.  It used to report whether memory
               for that aggregate has ever been allocated.  This
               behavior may disappear in future versions of Perl.
               You should instead use a simple test for size:

                   if (@an_array) { print "has array elements\n" }
                   if (%a_hash)   { print "has hash members\n"   }

               When used on a hash element, it tells you whether
               the value is defined, not whether the key exists
               in the hash.  Use "exists" for the latter purpose.


                   print if defined $switch{'D'};
                   print "$val\n" while defined($val = pop(@ary));
                   die "Can't readlink $sym: $!"
                       unless defined($value = readlink $sym);
                   sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
                   $debugging = 0 unless defined $debugging;

               Note:  Many folks tend to overuse "defined", and
               then are surprised to discover that the number 0
               and "" (the zero-length string) are, in fact,
               defined values.  For example, if you say

                   "ab" =~ /a(.*)b/;

               The pattern match succeeds, and $1 is defined,
               despite the fact that it matched "nothing".  But
               it didn't really match nothing--rather, it matched
               something that happened to be zero characters
               long.  This is all very above-board and honest.
               When a function returns an undefined value, it's
               an admission that it couldn't give you an honest
               answer.  So you should use "defined" only when
               you're questioning the integrity of what you're
               trying to do.  At other times, a simple comparison
               value if there was no such element.  Deleting from
               $ENV{} modifies the environment.  Deleting from a
               hash tied to a DBM file deletes the entry from the
               DBM file.  Deleting from a "tie"d hash or array
               may not necessarily return anything.

               Deleting an array element effectively returns that
               position of the array to its initial, uninitial­
               ized state.  Subsequently testing for the same
               element with exists() will return false.  Note
               that deleting array elements in the middle of an
               array will not shift the index of the ones after
               them down--use splice() for that.  See "exists".

               The following (inefficiently) deletes all the val­
               ues of %HASH and @ARRAY:

                   foreach $key (keys %HASH) {
                       delete $HASH{$key};

                   foreach $index (0 .. $#ARRAY) {
                       delete $ARRAY[$index];

               And so do these:

                   delete @HASH{keys %HASH};

                   delete @ARRAY[0 .. $#ARRAY];

               But both of these are slower than just assigning
               the empty list or undefining %HASH or @ARRAY:

                   %HASH = ();         # completely empty %HASH
                   undef %HASH;        # forget %HASH ever existed

                   @ARRAY = ();        # completely empty @ARRAY
                   undef @ARRAY;       # forget @ARRAY ever existed

               Note that the EXPR can be arbitrarily complicated
               as long as the final operation is a hash element,
               array element,  hash slice, or array slice lookup:

                   delete $ref->[$x][$y]{$key};
                   delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

                   delete $ref->[$x][$y][$index];
                   delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

       die LIST
               Outside an "eval", prints the value of LIST to
               line, the current script line number and input
               line number (if any) are also printed, and a new­
               line is supplied.  Note that the "input line num­
               ber" (also known as "chunk") is subject to what­
               ever notion of "line" happens to be currently in
               effect, and is also available as the special vari­
               able $..  See "$/" in perlvar and "$." in perlvar.

               Hint: sometimes appending ", stopped" to your mes­
               sage will cause it to make better sense when the
               string "at foo line 123" is appended.  Suppose you
               are running script "canasta".

                   die "/etc/games is no good";
                   die "/etc/games is no good, stopped";

               produce, respectively

                   /etc/games is no good at canasta line 123.
                   /etc/games is no good, stopped at canasta line 123.

               See also exit(), warn(), and the Carp module.

               If LIST is empty and $@ already contains a value
               (typically from a previous eval) that value is
               reused after appending "\t...propagated".  This is
               useful for propagating exceptions:

                   eval { ... };
                   die unless $@ =~ /Expected exception/;

               If LIST is empty and $@ contains an object refer­
               ence that has a "PROPAGATE" method, that method
               will be called with additional file and line num­
               ber parameters.  The return value replaces the
               value in $@.  ie. as if "$@ = eval { $@->PROPA­
               GATE(__FILE__, __LINE__) };" were called.

               If $@ is empty then the string "Died" is used.

               die() can also be called with a reference argu­
               ment.  If this happens to be trapped within an
               eval(), $@ contains the reference.  This behavior
               permits a more elaborate exception handling imple­
               mentation using objects that maintain arbitrary
               state about the nature of the exception.  Such a
               scheme is sometimes preferable to matching partic­
               ular string values of $@ using regular expres­
               sions.  Here's an example:

                   eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
                   if ($@) {

               You can arrange for a callback to be run just
               before the "die" does its deed, by setting the
               $SIG{__DIE__} hook.  The associated handler will
               be called with the error text and can change the
               error message, if it sees fit, by calling "die"
               again.  See "$SIG{expr}" in perlvar for details on
               setting %SIG entries, and "eval BLOCK" for some
               examples.  Although this feature was meant to be
               run only right before your program was to exit,
               this is not currently the case--the $SIG{__DIE__}
               hook is currently called even inside eval()ed
               blocks/strings!  If one wants the hook to do noth­
               ing in such situations, put

                       die @_ if $^S;

               as the first line of the handler (see "$^S" in
               perlvar).  Because this promotes strange action at
               a distance, this counterintuitive behavior may be
               fixed in a future release.

       do BLOCK
               Not really a function.  Returns the value of the
               last command in the sequence of commands indicated
               by BLOCK.  When modified by a loop modifier, exe­
               cutes the BLOCK once before testing the loop con­
               dition.  (On other statements the loop modifiers
               test the conditional first.)

               "do BLOCK" does not count as a loop, so the loop
               control statements "next", "last", or "redo" can­
               not be used to leave or restart the block.  See
               perlsyn for alternative strategies.

               A deprecated form of subroutine call.  See perl­

       do EXPR Uses the value of EXPR as a filename and executes
               the contents of the file as a Perl script.  Its
               primary use is to include subroutines from a Perl
               subroutine library.

                   do 'stat.pl';

               is just like

                   eval `cat stat.pl`;

               except that it's more efficient and concise, keeps
               track of the current filename for error messages,
               searches the @INC libraries, and updates %INC if

               Note that inclusion of library modules is better
               done with the "use" and "require" operators, which
               also do automatic error checking and raise an
               exception if there's a problem.

               You might like to use "do" to read in a program
               configuration file.  Manual error checking can be
               done this way:

                   # read in config files: system first, then user
                   for $file ("/share/prog/defaults.rc",
                       unless ($return = do $file) {
                           warn "couldn't parse $file: $@" if $@;
                           warn "couldn't do $file: $!"    unless defined $return;
                           warn "couldn't run $file"       unless $return;

       dump LABEL
       dump    This function causes an immediate core dump.  See
               also the -u command-line switch in perlrun, which
               does the same thing.  Primarily this is so that
               you can use the undump program (not supplied) to
               turn your core dump into an executable binary
               after having initialized all your variables at the
               beginning of the program.  When the new binary is
               executed it will begin by executing a "goto LABEL"
               (with all the restrictions that "goto" suffers).
               Think of it as a goto with an intervening core
               dump and reincarnation.  If "LABEL" is omitted,
               restarts the program from the top.

               WARNING: Any files opened at the time of the dump
               will not be open any more when the program is
               reincarnated, with possible resulting confusion on
               the part of Perl.

               This function is now largely obsolete, partly
               because it's very hard to convert a core file into
               an executable, and because the real compiler back­
               ends for generating portable bytecode and compil­
               able C code have superseded it.  That's why you
               should now invoke it as "CORE::dump()", if you
               don't want to be warned against a possible typo.

               If you're looking to use dump to speed up your
               program, consider generating bytecode or native C
               code as described in perlcc.  If you're just try­
               ing to accelerate a CGI script, consider using the
               change in future versions of perl, but it is guar­
               anteed to be in the same order as either the
               "keys" or "values" function would produce on the
               same (unmodified) hash.  Since Perl 5.8.1 the
               ordering is different even between different runs
               of Perl for security reasons (see "Algorithmic
               Complexity Attacks" in perlsec).

               When the hash is entirely read, a null array is
               returned in list context (which when assigned pro­
               duces a false (0) value), and "undef" in scalar
               context.  The next call to "each" after that will
               start iterating again.  There is a single iterator
               for each hash, shared by all "each", "keys", and
               "values" function calls in the program; it can be
               reset by reading all the elements from the hash,
               or by evaluating "keys HASH" or "values HASH".  If
               you add or delete elements of a hash while you're
               iterating over it, you may get entries skipped or
               duplicated, so don't.  Exception: It is always
               safe to delete the item most recently returned by
               "each()", which means that the following code will

                       while (($key, $value) = each %hash) {
                         print $key, "\n";
                         delete $hash{$key};   # This is safe

               The following prints out your environment like the
               printenv(1) program, only in a different order:

                   while (($key,$value) = each %ENV) {
                       print "$key=$value\n";

               See also "keys", "values" and "sort".

       eof FILEHANDLE
       eof ()
       eof     Returns 1 if the next read on FILEHANDLE will
               return end of file, or if FILEHANDLE is not open.
               FILEHANDLE may be an expression whose value gives
               the real filehandle.  (Note that this function
               actually reads a character and then "ungetc"s it,
               so isn't very useful in an interactive context.)
               Do not read from a terminal file (or call
               "eof(FILEHANDLE)" on it) after end-of-file is
               reached.  File types such as terminals may lose
               the end-of-file condition if you do.

               An "eof" without an argument uses the last file
               be used to detect the end of each file, "eof()"
               will only detect the end of the last file.  Exam­

                   # reset line numbering on each input file
                   while (<>) {
                       next if /^\s*#/;        # skip comments
                       print "$.\t$_";
                   } continue {
                       close ARGV  if eof;     # Not eof()!

                   # insert dashes just before last line of last file
                   while (<>) {
                       if (eof()) {            # check for end of last file
                           print "--------------\n";
                       last if eof();          # needed if we're reading from a terminal

               Practical hint: you almost never need to use "eof"
               in Perl, because the input operators typically
               return "undef" when they run out of data, or if
               there was an error.

       eval EXPR
       eval BLOCK
               In the first form, the return value of EXPR is
               parsed and executed as if it were a little Perl
               program.  The value of the expression (which is
               itself determined within scalar context) is first
               parsed, and if there weren't any errors, executed
               in the lexical context of the current Perl pro­
               gram, so that any variable settings or subroutine
               and format definitions remain afterwards.  Note
               that the value is parsed every time the eval exe­
               cutes.  If EXPR is omitted, evaluates $_.  This
               form is typically used to delay parsing and subse­
               quent execution of the text of EXPR until run

               In the second form, the code within the BLOCK is
               parsed only once--at the same time the code sur­
               rounding the eval itself was parsed--and executed
               within the context of the current Perl program.
               This form is typically used to trap exceptions
               more efficiently than the first (see below), while
               also providing the benefit of checking the code
               within BLOCK at compile time.

               The final semicolon, if any, may be omitted from
               message.  If there was no error, $@ is guaranteed
               to be a null string.  Beware that using "eval"
               neither silences perl from printing warnings to
               STDERR, nor does it stuff the text of warning mes­
               sages into $@.  To do either of those, you have to
               use the $SIG{__WARN__} facility, or turn off warn­
               ings inside the BLOCK or EXPR using "no warn­
               ings 'all'".  See "warn", perlvar, warnings and

               Note that, because "eval" traps otherwise-fatal
               errors, it is useful for determining whether a
               particular feature (such as "socket" or "symlink")
               is implemented.  It is also Perl's exception trap­
               ping mechanism, where the die operator is used to
               raise exceptions.

               If the code to be executed doesn't vary, you may
               use the eval-BLOCK form to trap run-time errors
               without incurring the penalty of recompiling each
               time.  The error, if any, is still returned in $@.

                   # make divide-by-zero nonfatal
                   eval { $answer = $a / $b; }; warn $@ if $@;

                   # same thing, but less efficient
                   eval '$answer = $a / $b'; warn $@ if $@;

                   # a compile-time error
                   eval { $answer = };                 # WRONG

                   # a run-time error
                   eval '$answer =';   # sets $@

               Due to the current arguably broken state of
               "__DIE__" hooks, when using the "eval{}" form as
               an exception trap in libraries, you may wish not
               to trigger any "__DIE__" hooks that user code may
               have installed.  You can use the "local
               $SIG{__DIE__}" construct for this purpose, as
               shown in this example:

                   # a very private exception trap for divide-by-zero
                   eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
                   warn $@ if $@;

               This is especially significant, given that
               "__DIE__" hooks can call "die" again, which has
               the effect of changing their error messages:

                   # __DIE__ hooks may modify error messages
                   eval $x;            # CASE 1
                   eval "$x";          # CASE 2

                   eval '$x';          # CASE 3
                   eval { $x };        # CASE 4

                   eval "\$$x++";      # CASE 5
                   $$x++;              # CASE 6

               Cases 1 and 2 above behave identically: they run
               the code contained in the variable $x.  (Although
               case 2 has misleading double quotes making the
               reader wonder what else might be happening (noth­
               ing is).)  Cases 3 and 4 likewise behave in the
               same way: they run the code '$x', which does noth­
               ing but return the value of $x.  (Case 4 is pre­
               ferred for purely visual reasons, but it also has
               the advantage of compiling at compile-time instead
               of at run-time.)  Case 5 is a place where normally
               you would like to use double quotes, except that
               in this particular situation, you can just use
               symbolic references instead, as in case 6.

               "eval BLOCK" does not count as a loop, so the loop
               control statements "next", "last", or "redo" can­
               not be used to leave or restart the block.

               Note that as a very special case, an "eval ''"
               executed within the "DB" package doesn't see the
               usual surrounding lexical scope, but rather the
               scope of the first non-DB piece of code that
               called it. You don't normally need to worry about
               this unless you are writing a Perl debugger.

       exec LIST
       exec PROGRAM LIST
               The "exec" function executes a system command and
               never returns-- use "system" instead of "exec" if
               you want it to return.  It fails and returns false
               only if the command does not exist and it is exe­
               cuted directly instead of via your system's com­
               mand shell (see below).

               Since it's a common mistake to use "exec" instead
               of "system", Perl warns you if there is a follow­
               ing statement which isn't "die", "warn", or "exit"
               (if "-w" is set  -  but you always do that).   If
               you really want to follow an "exec" with some
               other statement, you can use one of these styles
               to avoid the warning:

                   exec ('foo')   or print STDERR "couldn't exec foo: $!";

                   exec '/bin/echo', 'Your arguments are: ', @ARGV;
                   exec "sort $outfile | uniq";

               If you don't really want to execute the first
               argument, but want to lie to the program you are
               executing about its own name, you can specify the
               program you actually want to run as an "indirect
               object" (without a comma) in front of the LIST.
               (This always forces interpretation of the LIST as
               a multivalued list, even if there is only a single
               scalar in the list.)  Example:

                   $shell = '/bin/csh';
                   exec $shell '-sh';          # pretend it's a login shell

               or, more directly,

                   exec {'/bin/csh'} '-sh';    # pretend it's a login shell

               When the arguments get executed via the system
               shell, results will be subject to its quirks and
               capabilities.  See "`STRING`" in perlop for

               Using an indirect object with "exec" or "system"
               is also more secure.  This usage (which also works
               fine with system()) forces interpretation of the
               arguments as a multivalued list, even if the list
               had just one argument.  That way you're safe from
               the shell expanding wildcards or splitting up
               words with whitespace in them.

                   @args = ( "echo surprise" );

                   exec @args;               # subject to shell escapes
                                               # if @args == 1
                   exec { $args[0] } @args;  # safe even with one-arg list

               The first version, the one without the indirect
               object, ran the echo program, passing it "sur­
               prise" an argument.  The second version didn't--it
               tried to run a program literally called "echo sur­
               prise", didn't find it, and set $? to a non-zero
               value indicating failure.

               Beginning with v5.6.0, Perl will attempt to flush
               all files opened for output before the exec, but
               this may not be supported on some platforms (see
               perlport).  To be safe, you may need to set $|
               ($AUTOFLUSH in English) or call the "autoflush()"
               method of "IO::Handle" on any open handles in
                   print "Exists\n"    if exists $hash{$key};
                   print "Defined\n"   if defined $hash{$key};
                   print "True\n"      if $hash{$key};

                   print "Exists\n"    if exists $array[$index];
                   print "Defined\n"   if defined $array[$index];
                   print "True\n"      if $array[$index];

               A hash or array element can be true only if it's
               defined, and defined if it exists, but the reverse
               doesn't necessarily hold true.

               Given an expression that specifies the name of a
               subroutine, returns true if the specified subrou­
               tine has ever been declared, even if it is unde­
               fined.  Mentioning a subroutine name for exists or
               defined does not count as declaring it.  Note that
               a subroutine which does not exist may still be
               callable: its package may have an "AUTOLOAD"
               method that makes it spring into existence the
               first time that it is called -- see perlsub.

                   print "Exists\n"    if exists &subroutine;
                   print "Defined\n"   if defined &subroutine;

               Note that the EXPR can be arbitrarily complicated
               as long as the final operation is a hash or array
               key lookup or subroutine name:

                   if (exists $ref->{A}->{B}->{$key})  { }
                   if (exists $hash{A}{B}{$key})       { }

                   if (exists $ref->{A}->{B}->[$ix])   { }
                   if (exists $hash{A}{B}[$ix])        { }

                   if (exists &{$ref->{A}{B}{$key}})   { }

               Although the deepest nested array or hash will not
               spring into existence just because its existence
               was tested, any intervening ones will.  Thus
               "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring
               into existence due to the existence test for the
               $key element above.  This happens anywhere the
               arrow operator is used, including even:

                   undef $ref;
                   if (exists $ref->{"Some key"})      { }
                   print $ref;             # prints HASH(0x80d3d5c)

               This surprising autovivification in what does not
               at first--or even second--glance appear to be an
               lvalue context may be fixed in a future release.

                   $ans = <STDIN>;
                   exit 0 if $ans =~ /^[Xx]/;

               See also "die".  If EXPR is omitted, exits with 0
               status.  The only universally recognized values
               for EXPR are 0 for success and 1 for error; other
               values are subject to interpretation depending on
               the environment in which the Perl program is run­
               ning.  For example, exiting 69 (EX_UNAVAILABLE)
               from a sendmail incoming-mail filter will cause
               the mailer to return the item undelivered, but
               that's not true everywhere.

               Don't use "exit" to abort a subroutine if there's
               any chance that someone might want to trap what­
               ever error happened.  Use "die" instead, which can
               be trapped by an "eval".

               The exit() function does not always exit immedi­
               ately.  It calls any defined "END" routines first,
               but these "END" routines may not themselves abort
               the exit.  Likewise any object destructors that
               need to be called are called before the real exit.
               If this is a problem, you can call
               "POSIX:_exit($status)" to avoid END and destructor
               processing.  See perlmod for details.

       exp EXPR
       exp     Returns e (the natural logarithm base) to the
               power of EXPR.  If EXPR is omitted, gives

               Implements the fcntl(2) function.  You'll probably
               have to say

                   use Fcntl;

               first to get the correct constant definitions.
               Argument processing and value return works just
               like "ioctl" below.  For example:

                   use Fcntl;
                   fcntl($filehandle, F_GETFL, $packed_return_buffer)
                       or die "can't fcntl F_GETFL: $!";

               You don't have to check for "defined" on the
               return from "fcntl".  Like "ioctl", it maps a 0
               return from the system call into "0 but true" in
               Perl.  This string is true in boolean context and
               0 in numeric context.  It is also exempt from the
               is taken as an indirect filehandle, generally its

               You can use this to find out whether two handles
               refer to the same underlying descriptor:

                   if (fileno(THIS) == fileno(THAT)) {
                       print "THIS and THAT are dups\n";

               (Filehandles connected to memory objects via new
               features of "open" may return undefined even
               though they are open.)

               Calls flock(2), or an emulation of it, on FILEHAN­
               DLE.  Returns true for success, false on failure.
               Produces a fatal error if used on a machine that
               doesn't implement flock(2), fcntl(2) locking, or
               lockf(3).  "flock" is Perl's portable file locking
               interface, although it locks only entire files,
               not records.

               Two potentially non-obvious but traditional
               "flock" semantics are that it waits indefinitely
               until the lock is granted, and that its locks
               merely advisory.  Such discretionary locks are
               more flexible, but offer fewer guarantees.  This
               means that files locked with "flock" may be modi­
               fied by programs that do not also use "flock".
               See perlport, your port's specific documentation,
               or your system-specific local manpages for
               details.  It's best to assume traditional behavior
               if you're writing portable programs.  (But if
               you're not, you should as always feel perfectly
               free to write for your own system's idiosyncrasies
               (sometimes called "features").  Slavish adherence
               to portability concerns shouldn't get in the way
               of your getting your job done.)

               OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN,
               possibly combined with LOCK_NB.  These constants
               are traditionally valued 1, 2, 8 and 4, but you
               can use the symbolic names if you import them from
               the Fcntl module, either individually, or as a
               group using the ':flock' tag.  LOCK_SH requests a
               shared lock, LOCK_EX requests an exclusive lock,
               and LOCK_UN releases a previously requested lock.
               If LOCK_NB is bitwise-or'ed with LOCK_SH or
               LOCK_EX then "flock" will return immediately
               rather than blocking waiting for the lock (check
               the return status to see if you got it).
               requires that FILEHANDLE be open with read intent
               to use LOCK_SH and requires that it be open with
               write intent to use LOCK_EX.

               Note also that some versions of "flock" cannot
               lock things over the network; you would need to
               use the more system-specific "fcntl" for that.  If
               you like you can force Perl to ignore your sys­
               tem's flock(2) function, and so provide its own
               fcntl(2)-based emulation, by passing the switch
               "-Ud_flock" to the Configure program when you con­
               figure perl.

               Here's a mailbox appender for BSD systems.

                   use Fcntl ':flock'; # import LOCK_* constants

                   sub lock {
                       # and, in case someone appended
                       # while we were waiting...
                       seek(MBOX, 0, 2);

                   sub unlock {

                   open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
                           or die "Can't open mailbox: $!";

                   print MBOX $msg,"\n\n";

               On systems that support a real flock(), locks are
               inherited across fork() calls, whereas those that
               must resort to the more capricious fcntl() func­
               tion lose the locks, making it harder to write

               See also DB_File for other flock() examples.

       fork    Does a fork(2) system call to create a new process
               running the same program at the same point.  It
               returns the child pid to the parent process, 0 to
               the child process, or "undef" if the fork is
               unsuccessful.  File descriptors (and sometimes
               locks on those descriptors) are shared, while
               everything else is copied.  On most systems sup­
               porting fork(), great care has gone into making it
               extremely efficient (for example, using copy-on-
               tems, you can avoid this by setting $SIG{CHLD} to
               "IGNORE".  See also perlipc for more examples of
               forking and reaping moribund children.

               Note that if your forked child inherits system
               file descriptors like STDIN and STDOUT that are
               actually connected by a pipe or socket, even if
               you exit, then the remote server (such as, say, a
               CGI script or a backgrounded job launched from a
               remote shell) won't think you're done.  You should
               reopen those to /dev/null if it's any issue.

       format  Declare a picture format for use by the "write"
               function.  For example:

                   format Something =
                       Test: @<<<<<<<< @||||| @>>>>>
                             $str,     $%,    '$' . int($num)

                   $str = "widget";
                   $num = $cost/$quantity;
                   $~ = 'Something';

               See perlform for many details and examples.

       formline PICTURE,LIST
               This is an internal function used by "format"s,
               though you may call it, too.  It formats (see
               perlform) a list of values according to the con­
               tents of PICTURE, placing the output into the for­
               mat output accumulator, $^A (or $ACCUMULATOR in
               English).  Eventually, when a "write" is done, the
               contents of $^A are written to some filehandle,
               but you could also read $^A yourself and then set
               $^A back to "".  Note that a format typically does
               one "formline" per line of form, but the "form­
               line" function itself doesn't care how many new­
               lines are embedded in the PICTURE.  This means
               that the "~" and "~~" tokens will treat the entire
               PICTURE as a single line.  You may therefore need
               to use multiple formlines to implement a single
               record format, just like the format compiler.

               Be careful if you put double quotes around the
               picture, because an "@" character may be taken to
               mean the beginning of an array name.  "formline"
               always returns true.  See perlform for other exam­

       getc FILEHANDLE
                       system "stty", '-icanon', 'eol', "\001";

                   $key = getc(STDIN);

                   if ($BSD_STYLE) {
                       system "stty -cbreak </dev/tty >/dev/tty 2>&1";
                   else {
                       system "stty", 'icanon', 'eol', '^@'; # ASCII null
                   print "\n";

               Determination of whether $BSD_STYLE should be set
               is left as an exercise to the reader.

               The "POSIX::getattr" function can do this more
               portably on systems purporting POSIX compliance.
               See also the "Term::ReadKey" module from your
               nearest CPAN site; details on CPAN can be found on
               "CPAN" in perlmodlib.

               Implements the C library function of the same
               name, which on most systems returns the current
               login from /etc/utmp, if any.  If null, use "getp­

                   $login = getlogin || getpwuid($<) || "Kilroy";

               Do not consider "getlogin" for authentication: it
               is not as secure as "getpwuid".

       getpeername SOCKET
               Returns the packed sockaddr address of other end
               of the SOCKET connection.

                   use Socket;
                   $hersockaddr    = getpeername(SOCK);
                   ($port, $iaddr) = sockaddr_in($hersockaddr);
                   $herhostname    = gethostbyaddr($iaddr, AF_INET);
                   $herstraddr     = inet_ntoa($iaddr);

       getpgrp PID
               Returns the current process group for the speci­
               fied PID.  Use a PID of 0 to get the current pro­
               cess group for the current process.  Will raise an
               exception if used on a machine that doesn't imple­
               ment getpgrp(2).  If PID is omitted, returns pro­
               cess group of current process.  Note that the
               POSIX version of "getpgrp" does not accept a PID
               argument, so only "PID==0" is truly portable.
               cess group, or a user.  (See getpriority(2).)
               Will raise a fatal exception if used on a machine
               that doesn't implement getpriority(2).

       getpwnam NAME
       getgrnam NAME
       gethostbyname NAME
       getnetbyname NAME
       getprotobyname NAME
       getpwuid UID
       getgrgid GID
       getservbyname NAME,PROTO
       gethostbyaddr ADDR,ADDRTYPE
       getnetbyaddr ADDR,ADDRTYPE
       getprotobynumber NUMBER
       getservbyport PORT,PROTO
       sethostent STAYOPEN
       setnetent STAYOPEN
       setprotoent STAYOPEN
       setservent STAYOPEN
               These routines perform the same functions as their
               counterparts in the system library.  In list con­
               text, the return values from the various get rou­
               tines are as follows:

                      $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
                   ($name,$passwd,$gid,$members) = getgr*
                   ($name,$aliases,$addrtype,$length,@addrs) = gethost*
                   ($name,$aliases,$addrtype,$net) = getnet*
                   ($name,$aliases,$proto) = getproto*
                   ($name,$aliases,$port,$proto) = getserv*

               (If the entry doesn't exist you get a null list.)

               The exact meaning of the $gcos field varies but it
               usually contains the real name of the user (as
               opposed to the login name) and other information
                   $uid   = getpwnam($name);
                   $name  = getpwuid($num);
                   $name  = getpwent();
                   $gid   = getgrnam($name);
                   $name  = getgrgid($num);
                   $name  = getgrent();

               In getpw*() the fields $quota, $comment, and
               $expire are special cases in the sense that in
               many systems they are unsupported.  If the $quota
               is unsupported, it is an empty scalar.  If it is
               supported, it usually encodes the disk quota.  If
               the $comment field is unsupported, it is an empty
               scalar.  If it is supported it usually encodes
               some administrative comment about the user.  In
               some systems the $quota field may be $change or
               $age, fields that have to do with password aging.
               In some systems the $comment field may be $class.
               The $expire field, if present, encodes the expira­
               tion period of the account or the password.  For
               the availability and the exact meaning of these
               fields in your system, please consult your getpw­
               nam(3) documentation and your pwd.h file.  You can
               also find out from within Perl what your $quota
               and $comment fields mean and whether you have the
               $expire field by using the "Config" module and the
               values "d_pwquota", "d_pwage", "d_pwchange",
               "d_pwcomment", and "d_pwexpire".  Shadow password
               files are only supported if your vendor has imple­
               mented them in the intuitive fashion that calling
               the regular C library routines gets the shadow
               versions if you're running under privilege or if
               there exists the shadow(3) functions as found in
               System V ( this includes Solaris and Linux.)
               Those systems which implement a proprietary shadow
               password facility are unlikely to be supported.

               The $members value returned by getgr*() is a space
               separated list of the login names of the members
               of the group.

               For the gethost*() functions, if the "h_errno"
               variable is supported in C, it will be returned to
               you via $? if the function call fails.  The @addrs
               value returned by a successful call is a list of
               the raw addresses returned by the corresponding
               system library call.  In the Internet domain, each
               address is four bytes long and you can unpack it
               by saying something like:

                   ($a,$b,$c,$d) = unpack('C4',$addr[0]);

               "Net::protoent", "Net::servent", "Time::gmtime",
               "Time::localtime", and "User::grent".  These over­
               ride the normal built-ins, supplying versions that
               return objects with the appropriate names for each
               field.  For example:

                  use File::stat;
                  use User::pwent;
                  $is_his = (stat($filename)->uid == pwent($whoever)->uid);

               Even though it looks like they're the same method
               calls (uid), they aren't, because a "File::stat"
               object is different from a "User::pwent" object.

       getsockname SOCKET
               Returns the packed sockaddr address of this end of
               the SOCKET connection, in case you don't know the
               address because you have several different IPs
               that the connection might have come in on.

                   use Socket;
                   $mysockaddr = getsockname(SOCK);
                   ($port, $myaddr) = sockaddr_in($mysockaddr);
                   printf "Connect to %s [%s]\n",
                      scalar gethostbyaddr($myaddr, AF_INET),

       getsockopt SOCKET,LEVEL,OPTNAME
               Returns the socket option requested, or undef if
               there is an error.

       glob EXPR
       glob    In list context, returns a (possibly empty) list
               of filename expansions on the value of EXPR such
               as the standard Unix shell /bin/csh would do. In
               scalar context, glob iterates through such file­
               name expansions, returning undef when the list is
               exhausted. This is the internal function imple­
               menting the "<*.c>" operator, but you can use it
               directly. If EXPR is omitted, $_ is used.  The
               "<*.c>" operator is discussed in more detail in
               "I/O Operators" in perlop.

               Beginning with v5.6.0, this operator is imple­
               mented using the standard "File::Glob" extension.
               See File::Glob for details.

       gmtime EXPR
               Converts a time as returned by the time function
               to an 8-element list with the time localized for
               the standard Greenwich time zone.  Typically used
               as follows:
               ing Wednesday.  $yday is the day of the year, in
               the range 0..364 (or 0..365 in leap years.)

               Note that the $year element is not simply the last
               two digits of the year.  If you assume it is, then
               you create non-Y2K-compliant programs--and you
               wouldn't want to do that, would you?

               The proper way to get a complete 4-digit year is

                       $year += 1900;

               And to get the last two digits of the year (e.g.,
               '01' in 2001) do:

                       $year = sprintf("%02d", $year % 100);

               If EXPR is omitted, "gmtime()" uses the current
               time ("gmtime(time)").

               In scalar context, "gmtime()" returns the ctime(3)

                   $now_string = gmtime;  # e.g., "Thu Oct 13 04:54:34 1994"

               Also see the "timegm" function provided by the
               "Time::Local" module, and the strftime(3) function
               available via the POSIX module.

               This scalar value is not locale dependent (see
               perllocale), but is instead a Perl builtin.  Also
               see the "Time::Local" module, and the strftime(3)
               and mktime(3) functions available via the POSIX
               module.  To get somewhat similar but locale depen­
               dent date strings, set up your locale environment
               variables appropriately (please see perllocale)
               and try for example:

                   use POSIX qw(strftime);
                   $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

               Note that the %a and %b escapes, which represent
               the short forms of the day of the week and the
               month of the year, may not necessarily be three
               characters wide in all locales.

       goto LABEL
       goto EXPR
       goto &NAME
               The "goto-LABEL" form finds the statement labeled
               with LABEL and resumes execution there.  It may

               The "goto-EXPR" form expects a label name, whose
               scope will be resolved dynamically.  This allows
               for computed "goto"s per FORTRAN, but isn't neces­
               sarily recommended if you're optimizing for main­

                   goto ("FOO", "BAR", "GLARCH")[$i];

               The "goto-&NAME" form is quite different from the
               other forms of "goto".  In fact, it isn't a goto
               in the normal sense at all, and doesn't have the
               stigma associated with other gotos.  Instead, it
               exits the current subroutine (losing any changes
               set by local()) and immediately calls in its place
               the named subroutine using the current value of
               @_.  This is used by "AUTOLOAD" subroutines that
               wish to load another subroutine and then pretend
               that the other subroutine had been called in the
               first place (except that any modifications to @_
               in the current subroutine are propagated to the
               other subroutine.)  After the "goto", not even
               "caller" will be able to tell that this routine
               was called first.

               NAME needn't be the name of a subroutine; it can
               be a scalar variable containing a code reference,
               or a block which evaluates to a code reference.

       grep BLOCK LIST
       grep EXPR,LIST
               This is similar in spirit to, but not the same as,
               grep(1) and its relatives.  In particular, it is
               not limited to using regular expressions.

               Evaluates the BLOCK or EXPR for each element of
               LIST (locally setting $_ to each element) and
               returns the list value consisting of those ele­
               ments for which the expression evaluated to true.
               In scalar context, returns the number of times the
               expression was true.

                   @foo = grep(!/^#/, @bar);    # weed out comments

               or equivalently,

                   @foo = grep {!/^#/} @bar;    # weed out comments

               Note that $_ is an alias to the list value, so it
               can be used to modify the elements of the LIST.
               While this is useful and supported, it can cause

       hex     Interprets EXPR as a hex string and returns the
               corresponding value.  (To convert strings that
               might start with either 0, 0x, or 0b, see "oct".)
               If EXPR is omitted, uses $_.

                   print hex '0xAf'; # prints '175'
                   print hex 'aF';   # same

               Hex strings may only represent integers.  Strings
               that would cause integer overflow trigger a warn­
               ing.  Leading whitespace is not stripped, unlike

       import  There is no builtin "import" function.  It is just
               an ordinary method (subroutine) defined (or inher­
               ited) by modules that wish to export names to
               another module.  The "use" function calls the
               "import" method for the package used.  See also
               "use", perlmod, and Exporter.

       index STR,SUBSTR
               The index function searches for one string within
               another, but without the wildcard-like behavior of
               a full regular-expression pattern match.  It
               returns the position of the first occurrence of
               SUBSTR in STR at or after POSITION.  If POSITION
               is omitted, starts searching from the beginning of
               the string.  The return value is based at 0 (or
               whatever you've set the $[ variable to--but don't
               do that).  If the substring is not found, returns
               one less than the base, ordinarily "-1".

       int EXPR
       int     Returns the integer portion of EXPR.  If EXPR is
               omitted, uses $_.  You should not use this func­
               tion for rounding: one because it truncates
               towards 0, and two because machine representations
               of floating point numbers can sometimes produce
               counterintuitive results.  For example,
               "int(-6.725/0.025)" produces -268 rather than the
               correct -269; that's because it's really more like
               -268.99999999999994315658 instead.  Usually, the
               "sprintf", "printf", or the "POSIX::floor" and
               "POSIX::ceil" functions will serve you better than
               will int().

               Implements the ioctl(2) function.  You'll probably
               first have to say

                   require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph

               add a 0 to the scalar before using it.)  The
               "pack" and "unpack" functions may be needed to
               manipulate the values of structures used by

               The return value of "ioctl" (and "fcntl") is as

                       if OS returns:          then Perl returns:
                           -1                    undefined value
                            0                  string "0 but true"
                       anything else               that number

               Thus Perl returns true on success and false on
               failure, yet you can still easily determine the
               actual value returned by the operating system:

                   $retval = ioctl(...) || -1;
                   printf "System returned %d\n", $retval;

               The special string "0 but true" is exempt from -w
               complaints about improper numeric conversions.

               Here's an example of setting a filehandle named
               "REMOTE" to be non-blocking at the system level.
               You'll have to negotiate $| on your own, though.

                   use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

                   $flags = fcntl(REMOTE, F_GETFL, 0)
                               or die "Can't get flags for the socket: $!\n";

                   $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
                               or die "Can't set flags for the socket: $!\n";

       join EXPR,LIST
               Joins the separate strings of LIST into a single
               string with fields separated by the value of EXPR,
               and returns that new string.  Example:

                   $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

               Beware that unlike "split", "join" doesn't take a
               pattern as its first argument.  Compare "split".

       keys HASH
               Returns a list consisting of all the keys of the
               named hash.  (In scalar context, returns the num­
               ber of keys.)

               The keys are returned in an apparently random
               order.  The actual random order is subject to
                   @values = values %ENV;
                   while (@keys) {
                       print pop(@keys), '=', pop(@values), "\n";

               or how about sorted by key:

                   foreach $key (sort(keys %ENV)) {
                       print $key, '=', $ENV{$key}, "\n";

               The returned values are copies of the original
               keys in the hash, so modifying them will not
               affect the original hash.  Compare "values".

               To sort a hash by value, you'll need to use a
               "sort" function.  Here's a descending numeric sort
               of a hash by its values:

                   foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
                       printf "%4d %s\n", $hash{$key}, $key;

               As an lvalue "keys" allows you to increase the
               number of hash buckets allocated for the given
               hash.  This can gain you a measure of efficiency
               if you know the hash is going to get big.  (This
               is similar to pre-extending an array by assigning
               a larger number to $#array.)  If you say

                   keys %hash = 200;

               then %hash will have at least 200 buckets allo­
               cated for it--256 of them, in fact, since it
               rounds up to the next power of two.  These buckets
               will be retained even if you do "%hash = ()", use
               "undef %hash" if you want to free the storage
               while %hash is still in scope.  You can't shrink
               the number of buckets allocated for the hash using
               "keys" in this way (but you needn't worry about
               doing this by accident, as trying has no effect).

               See also "each", "values" and "sort".

       kill SIGNAL, LIST
               Sends a signal to a list of processes.  Returns
               the number of processes successfully signaled
               (which is not necessarily the same as the number
               actually killed).

                   $cnt = kill 1, $child1, $child2;
                   kill 9, @goners;

               See "Signals" in perlipc for more details.

       last LABEL
       last    The "last" command is like the "break" statement
               in C (as used in loops); it immediately exits the
               loop in question.  If the LABEL is omitted, the
               command refers to the innermost enclosing loop.
               The "continue" block, if any, is not executed:

                   LINE: while (<STDIN>) {
                       last LINE if /^$/;      # exit when done with header

               "last" cannot be used to exit a block which
               returns a value such as "eval {}", "sub {}" or "do
               {}", and should not be used to exit a grep() or
               map() operation.

               Note that a block by itself is semantically iden­
               tical to a loop that executes once.  Thus "last"
               can be used to effect an early exit out of such a

               See also "continue" for an illustration of how
               "last", "next", and "redo" work.

       lc EXPR
       lc      Returns a lowercased version of EXPR.  This is the
               internal function implementing the "\L" escape in
               double-quoted strings.  Respects current LC_CTYPE
               locale if "use locale" in force.  See perllocale
               and perlunicode for more details about locale and
               Unicode support.

               If EXPR is omitted, uses $_.

       lcfirst EXPR
       lcfirst Returns the value of EXPR with the first character
               lowercased.  This is the internal function imple­
               menting the "\l" escape in double-quoted strings.
               Respects current LC_CTYPE locale if "use locale"
               in force.  See perllocale and perlunicode for more
               details about locale and Unicode support.

               If EXPR is omitted, uses $_.

       length EXPR
       length  Returns the length in characters of the value of
               EXPR.  If EXPR is omitted, returns length of $_.
               Note that this cannot be used on an entire array
               Does the same thing that the listen system call
               does.  Returns true if it succeeded, false other­
               wise.  See the example in "Sockets: Client/Server
               Communication" in perlipc.

       local EXPR
               You really probably want to be using "my" instead,
               because "local" isn't what most people think of as
               "local".  See "Private Variables via my()" in
               perlsub for details.

               A local modifies the listed variables to be local
               to the enclosing block, file, or eval.  If more
               than one value is listed, the list must be placed
               in parentheses.  See "Temporary Values via
               local()" in perlsub for details, including issues
               with tied arrays and hashes.

       localtime EXPR
               Converts a time as returned by the time function
               to a 9-element list with the time analyzed for the
               local time zone.  Typically used as follows:

                   #  0    1    2     3     4    5     6     7     8
                   ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =

               All list elements are numeric, and come straight
               out of the C `struct tm'.  $sec, $min, and $hour
               are the seconds, minutes, and hours of the speci­
               fied time.  $mday is the day of the month, and
               $mon is the month itself, in the range 0..11 with
               0 indicating January and 11 indicating December.
               $year is the number of years since 1900.  That is,
               $year is 123 in year 2023.  $wday is the day of
               the week, with 0 indicating Sunday and 3 indicat­
               ing Wednesday.  $yday is the day of the year, in
               the range 0..364 (or 0..365 in leap years.)
               $isdst is true if the specified time occurs during
               daylight savings time, false otherwise.

               Note that the $year element is not simply the last
               two digits of the year.  If you assume it is, then
               you create non-Y2K-compliant programs--and you
               wouldn't want to do that, would you?

               The proper way to get a complete 4-digit year is

                       $year += 1900;

               And to get the last two digits of the year (e.g.,
               the "Time::Local" module (to convert the second,
               minutes, hours, ... back to seconds since the
               stroke of midnight the 1st of January 1970, the
               value returned by time()), and the strftime(3) and
               mktime(3) functions available via the POSIX mod­
               ule.  To get somewhat similar but locale dependent
               date strings, set up your locale environment vari­
               ables appropriately (please see perllocale) and
               try for example:

                   use POSIX qw(strftime);
                   $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;

               Note that the %a and %b, the short forms of the
               day of the week and the month of the year, may not
               necessarily be three characters wide.

       lock THING
               This function places an advisory lock on a shared
               variable, or referenced object contained in THING
               until the lock goes out of scope.

               lock() is a "weak keyword" : this means that if
               you've defined a function by this name (before any
               calls to it), that function will be called
               instead. (However, if you've said "use threads",
               lock() is always a keyword.) See threads.

       log EXPR
       log     Returns the natural logarithm (base e) of EXPR.
               If EXPR is omitted, returns log of $_.  To get the
               log of another base, use basic algebra: The base-N
               log of a number is equal to the natural log of
               that number divided by the natural log of N.  For

                   sub log10 {
                       my $n = shift;
                       return log($n)/log(10);

               See also "exp" for the inverse operation.

       lstat EXPR
       lstat   Does the same thing as the "stat" function
               (including setting the special "_" filehandle) but
               stats a symbolic link instead of the file the sym­
               bolic link points to.  If symbolic links are unim­
               plemented on your system, a normal "stat" is done.
               For much more detailed information, please see the
               documentation for "stat".

                   @chars = map(chr, @nums);

               translates a list of numbers to the corresponding
               characters.  And

                   %hash = map { getkey($_) => $_ } @array;

               is just a funny way to write

                   %hash = ();
                   foreach $_ (@array) {
                       $hash{getkey($_)} = $_;

               Note that $_ is an alias to the list value, so it
               can be used to modify the elements of the LIST.
               While this is useful and supported, it can cause
               bizarre results if the elements of LIST are not
               variables.  Using a regular "foreach" loop for
               this purpose would be clearer in most cases.  See
               also "grep" for an array composed of those items
               of the original list for which the BLOCK or EXPR
               evaluates to true.

               "{" starts both hash references and blocks, so
               "map { ..." could be either the start of map BLOCK
               LIST or map EXPR, LIST. Because perl doesn't look
               ahead for the closing "}" it has to take a guess
               at which its dealing with based what it finds just
               after the "{". Usually it gets it right, but if it
               doesn't it won't realize something is wrong until
               it gets to the "}" and encounters the missing (or
               unexpected) comma. The syntax error will be
               reported close to the "}" but you'll need to
               change something near the "{" such as using a
               unary "+" to give perl some help:

                   %hash = map {  "\L$_", 1  } @array  # perl guesses EXPR.  wrong
                   %hash = map { +"\L$_", 1  } @array  # perl guesses BLOCK. right
                   %hash = map { ("\L$_", 1) } @array  # this also works
                   %hash = map {  lc($_), 1  } @array  # as does this.
                   %hash = map +( lc($_), 1 ), @array  # this is EXPR and works!

                   %hash = map  ( lc($_), 1 ), @array  # evaluates to (1, @array)

               or to force an anon hash constructor use "+{"

                  @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end

               and you get list of anonymous hashes each with
               only 1 entry.
               when the file or directory should be kept private
               (mail files, for instance).  The perlfunc(1) entry
               on "umask" discusses the choice of MASK in more

               Note that according to the POSIX 1003.1-1996 the
               FILENAME may have any number of trailing slashes.
               Some operating and filesystems do not get this
               right, so Perl automatically removes all trailing
               slashes to keep everyone happy.

       msgctl ID,CMD,ARG
               Calls the System V IPC function msgctl(2).  You'll
               probably have to say

                   use IPC::SysV;

               first to get the correct constant definitions.  If
               CMD is "IPC_STAT", then ARG must be a variable
               which will hold the returned "msqid_ds" structure.
               Returns like "ioctl": the undefined value for
               error, "0 but true" for zero, or the actual return
               value otherwise.  See also "SysV IPC" in perlipc,
               "IPC::SysV", and "IPC::Semaphore" documentation.

       msgget KEY,FLAGS
               Calls the System V IPC function msgget(2).
               Returns the message queue id, or the undefined
               value if there is an error.  See also "SysV IPC"
               in perlipc and "IPC::SysV" and "IPC::Msg" documen­

       msgrcv ID,VAR,SIZE,TYPE,FLAGS
               Calls the System V IPC function msgrcv to receive
               a message from message queue ID into variable VAR
               with a maximum message size of SIZE.  Note that
               when a message is received, the message type as a
               native long integer will be the first thing in
               VAR, followed by the actual message.  This packing
               may be opened with "unpack("l! a*")".  Taints the
               variable.  Returns true if successful, or false if
               there is an error.  See also "SysV IPC" in per­
               lipc, "IPC::SysV", and "IPC::SysV::Msg" documenta­

       msgsnd ID,MSG,FLAGS
               Calls the System V IPC function msgsnd to send the
               message MSG to the message queue ID.  MSG must
               begin with the native long integer message type,
               and be followed by the length of the actual mes­
               sage, and finally the message itself.  This kind
               of packing can be achieved with "pack("l! a*",
               ATTRS are still evolving.  TYPE is currently bound
               to the use of "fields" pragma, and attributes are
               handled using the "attributes" pragma, or starting
               from Perl 5.8.0 also via the "Attribute::Handlers"
               module.  See "Private Variables via my()" in perl­
               sub for details, and fields, attributes, and

       next LABEL
       next    The "next" command is like the "continue" state­
               ment in C; it starts the next iteration of the

                   LINE: while (<STDIN>) {
                       next LINE if /^#/;      # discard comments

               Note that if there were a "continue" block on the
               above, it would get executed even on discarded
               lines.  If the LABEL is omitted, the command
               refers to the innermost enclosing loop.

               "next" cannot be used to exit a block which
               returns a value such as "eval {}", "sub {}" or "do
               {}", and should not be used to exit a grep() or
               map() operation.

               Note that a block by itself is semantically iden­
               tical to a loop that executes once.  Thus "next"
               will exit such a block early.

               See also "continue" for an illustration of how
               "last", "next", and "redo" work.

       no Module VERSION LIST
       no Module VERSION
       no Module LIST
       no Module
               See the "use" function, which "no" is the opposite

       oct EXPR
       oct     Interprets EXPR as an octal string and returns the
               corresponding value.  (If EXPR happens to start
               off with "0x", interprets it as a hex string.  If
               EXPR starts off with "0b", it is interpreted as a
               binary string.  Leading whitespace is ignored in
               all three cases.)  The following will handle deci­
               mal, binary, octal, and hex in the standard Perl
               or C notation:

               automatic conversion assumes base 10.)

       open FILEHANDLE
               Opens the file whose filename is given by EXPR,
               and associates it with FILEHANDLE.

               (The following is a comprehensive reference to
               open(): for a gentler introduction you may con­
               sider perlopentut.)

               If FILEHANDLE is an undefined scalar variable (or
               array or hash element) the variable is assigned a
               reference to a new anonymous filehandle, otherwise
               if FILEHANDLE is an expression, its value is used
               as the name of the real filehandle wanted.  (This
               is considered a symbolic reference, so "use strict
               'refs'" should not be in effect.)

               If EXPR is omitted, the scalar variable of the
               same name as the FILEHANDLE contains the filename.
               (Note that lexical variables--those declared with
               "my"--will not work for this purpose; so if you're
               using "my", specify EXPR in your call to open.)

               If three or more arguments are specified then the
               mode of opening and the file name are separate. If
               MODE is '<' or nothing, the file is opened for
               input.  If MODE is '>', the file is truncated and
               opened for output, being created if necessary.  If
               MODE is '>>', the file is opened for appending,
               again being created if necessary.

               You can put a '+' in front of the '>' or '<' to
               indicate that you want both read and write access
               to the file; thus '+<' is almost always preferred
               for read/write updates--the '+>' mode would clob­
               ber the file first.  You can't usually use either
               read-write mode for updating textfiles, since they
               have variable length records.  See the -i switch
               in perlrun for a better approach.  The file is
               created with permissions of 0666 modified by the
               process' "umask" value.

               These various prefixes correspond to the fopen(3)
               modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.

               In the 2-arguments (and 1-argument) form of the
               call the mode and filename should be concatenated

               For three or more arguments if MODE is '|-', the
               filename is interpreted as a command to which out­
               put is to be piped, and if MODE is '-|', the file­
               name is interpreted as a command which pipes out­
               put to us.  In the 2-arguments (and 1-argument)
               form one should replace dash ('-') with the com­
               mand.  See "Using open() for IPC" in perlipc for
               more examples of this.  (You are not allowed to
               "open" to a command that pipes both in and out,
               but see IPC::Open2, IPC::Open3, and "Bidirectional
               Communication" in perlipc for alternatives.)

               In the three-or-more argument form of pipe opens,
               if LIST is specified (extra arguments after the
               command name) then LIST becomes arguments to the
               command invoked if the platform supports it.  The
               meaning of "open" with more than three arguments
               for non-pipe modes is not yet specified. Experi­
               mental "layers" may give extra LIST arguments

               In the 2-arguments (and 1-argument) form opening
               '-' opens STDIN and opening '>-' opens STDOUT.

               You may use the three-argument form of open to
               specify IO "layers" (sometimes also referred to as
               "disciplines") to be applied to the handle that
               affect how the input and output are processed (see
               open and PerlIO for more details). For example

                 open(FH, "<:utf8", "file")

               will open the UTF-8 encoded file containing Uni­
               code characters, see perluniintro. (Note that if
               layers are specified in the three-arg form then
               default layers set by the "open" pragma are

               Open returns nonzero upon success, the undefined
               value otherwise.  If the "open" involved a pipe,
               the return value happens to be the pid of the sub­

               If you're running Perl on a system that distin­
               guishes between text files and binary files, then
               you should check out "binmode" for tips for deal­
               ing with this.  The key distinction between sys­
               tems that need "binmode" and those that don't is
               their text file formats.  Systems like Unix, Mac
               OS, and Plan 9, which delimit lines with a single
               character, and which encode that character in C as
               As a special case the 3 arg form with a read/write
               mode and the third argument being "undef":

                   open(TMP, "+>", undef) or die ...

               opens a filehandle to an anonymous temporary file.
               Also using "+<" works for symmetry, but you really
               should consider writing something to the temporary
               file first.  You will need to seek() to do the

               File handles can be opened to "in memory" files
               held in Perl scalars via:

                   open($fh, '>', \$variable) || ..

               Though if you try to re-open "STDOUT" or "STDERR"
               as an "in memory" file, you have to close it

                   close STDOUT;
                   open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";


                   $ARTICLE = 100;
                   open ARTICLE or die "Can't find article $ARTICLE: $!\n";
                   while (<ARTICLE>) {...

                   open(LOG, '>>/usr/spool/news/twitlog');     # (log is reserved)
                   # if the open fails, output is discarded

                   open(DBASE, '+<', 'dbase.mine')             # open for update
                       or die "Can't open 'dbase.mine' for update: $!";

                   open(DBASE, '+<dbase.mine')                 # ditto
                       or die "Can't open 'dbase.mine' for update: $!";

                   open(ARTICLE, '-|', "caesar <$article")     # decrypt article
                       or die "Can't start caesar: $!";

                   open(ARTICLE, "caesar <$article |")         # ditto
                       or die "Can't start caesar: $!";

                   open(EXTRACT, "|sort >/tmp/Tmp$$")          # $$ is our process id
                       or die "Can't start sort: $!";

                   # in memory files
                   open(MEMORY,'>', \$var)
                       or die "Can't open memory file: $!";
                   print MEMORY "foo!\n";                      # output will end up in $var

                       local $_;
                       while (<$input>) {              # note use of indirection
                           if (/^#include "(.*)"/) {
                               process($1, $input);
                           #...                # whatever

               You may also, in the Bourne shell tradition, spec­
               ify an EXPR beginning with '>&', in which case the
               rest of the string is interpreted as the name of a
               filehandle (or file descriptor, if numeric) to be
               duped (as dup(2)) and opened.  You may use "&"
               after ">", ">>", "<", "+>", "+>>", and "+<".  The
               mode you specify should match the mode of the
               original filehandle.  (Duping a filehandle does
               not take into account any existing contents of IO
               buffers.) If you use the 3 arg form then you can
               pass either a number, the name of a filehandle or
               the normal "reference to a glob".

               Here is a script that saves, redirects, and
               restores "STDOUT" and "STDERR" using various meth­

                   open my $oldout, ">&STDOUT"     or die "Can't dup STDOUT: $!";
                   open OLDERR,     ">&", \*STDERR or die "Can't dup STDERR: $!";

                   open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
                   open STDERR, ">&STDOUT"     or die "Can't dup STDOUT: $!";

                   select STDERR; $| = 1;      # make unbuffered
                   select STDOUT; $| = 1;      # make unbuffered

                   print STDOUT "stdout 1\n";  # this works for
                   print STDERR "stderr 1\n";  # subprocesses too

                   close STDOUT;
                   close STDERR;

                   open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
                   open STDERR, ">&OLDERR"    or die "Can't dup OLDERR: $!";

                   print STDOUT "stdout 2\n";
                   print STDERR "stderr 2\n";

               If you specify '<&=X', where "X" is a file
               descriptor number or a filehandle, then Perl will
                   open(FH, ">&gt;&=", OLDFH)


                   open(FH, ">&gt;&=OLDFH")

               Being parsimonious on filehandles is also useful
               (besides being parsimonious) for example when
               something is dependent on file descriptors, like
               for example locking using flock().  If you do just
               "open(A, '>&gt;&B')", the filehandle A will not have
               the same file descriptor as B, and therefore
               flock(A) will not flock(B), and vice versa.  But
               with "open(A, '>&gt;&=B')" the filehandles will share
               the same file descriptor.

               Note that if you are using Perls older than 5.8.0,
               Perl will be using the standard C libraries'
               fdopen() to implement the "=" functionality.  On
               many UNIX systems fdopen() fails when file
               descriptors exceed a certain value, typically 255.
               For Perls 5.8.0 and later, PerlIO is most often
               the default.

               You can see whether Perl has been compiled with
               PerlIO or not by running "perl -V" and looking for
               "useperlio=" line.  If "useperlio" is "define",
               you have PerlIO, otherwise you don't.

               If you open a pipe on the command '-', i.e.,
               either '|-' or '-|' with 2-arguments (or 1-argu­
               ment) form of open(), then there is an implicit
               fork done, and the return value of open is the pid
               of the child within the parent process, and 0
               within the child process.  (Use "defined($pid)" to
               determine whether the open was successful.)  The
               filehandle behaves normally for the parent, but
               i/o to that filehandle is piped from/to the STD­
               OUT/STDIN of the child process.  In the child pro­
               cess the filehandle isn't opened--i/o happens
               from/to the new STDOUT or STDIN.  Typically this
               is used like the normal piped open when you want
               to exercise more control over just how the pipe
               command gets executed, such as when you are run­
               ning setuid, and don't want to have to scan shell
               commands for metacharacters.  The following
               triples are more or less equivalent:

                   open(FOO, "|tr '[a-z]' '[A-Z]'");
                   open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
                   open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
                   open(FOO, '|-', "tr", '[a-z]', '[A-Z]');

               Beginning with v5.6.0, Perl will attempt to flush
               all files opened for output before any operation
               that may do a fork, but this may not be supported
               on some platforms (see perlport).  To be safe, you
               may need to set $| ($AUTOFLUSH in English) or call
               the "autoflush()" method of "IO::Handle" on any
               open handles.

               On systems that support a close-on-exec flag on
               files, the flag will be set for the newly opened
               file descriptor as determined by the value of $^F.
               See "$^F" in perlvar.

               Closing any piped filehandle causes the parent
               process to wait for the child to finish, and
               returns the status value in $?.

               The filename passed to 2-argument (or 1-argument)
               form of open() will have leading and trailing
               whitespace deleted, and the normal redirection
               characters honored.  This property, known as
               "magic open", can often be used to good effect.  A
               user could specify a filename of "rsh cat file |",
               or you could change certain filenames as needed:

                   $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
                   open(FH, $filename) or die "Can't open $filename: $!";

               Use 3-argument form to open a file with arbitrary
               weird characters in it,

                   open(FOO, '<', $file);

               otherwise it's necessary to protect any leading
               and trailing whitespace:

                   $file =~ s#^(\s)#./$1#;
                   open(FOO, "< $file\0");

               (this may not work on some bizarre filesystems).
               One should conscientiously choose between the
               magic and 3-arguments form of open():

                   open IN, $ARGV[0];

               will allow the user to specify an argument of the
               form "rsh cat file |", but will not work on a
               filename which happens to have a trailing space,

                   open IN, '<', $ARGV[0];
                   $oldfh = select(HANDLE); $| = 1; select($oldfh);
                   print HANDLE "stuff $$\n";
                   seek(HANDLE, 0, 0);
                   print "File contains: ", <HANDLE>;

               Using the constructor from the "IO::Handle" pack­
               age (or one of its subclasses, such as "IO::File"
               or "IO::Socket"), you can generate anonymous file­
               handles that have the scope of whatever variables
               hold references to them, and automatically close
               whenever and however you leave that scope:

                   use IO::File;
                   sub read_myfile_munged {
                       my $ALL = shift;
                       my $handle = new IO::File;
                       open($handle, "myfile") or die "myfile: $!";
                       $first = <$handle>
                           or return ();     # Automatically closed here.
                       mung $first or die "mung failed";       # Or here.
                       return $first, <$handle> if $ALL;       # Or here.
                       $first;                                 # Or here.

               See "seek" for some details about mixing reading
               and writing.

       opendir DIRHANDLE,EXPR
               Opens a directory named EXPR for processing by
               "readdir", "telldir", "seekdir", "rewinddir", and
               "closedir".  Returns true if successful.  DIRHAN­
               DLE may be an expression whose value can be used
               as an indirect dirhandle, usually the real dirhan­
               dle name.  If DIRHANDLE is an undefined scalar
               variable (or array or hash element), the variable
               is assigned a reference to a new anonymous dirhan­
               dle.  DIRHANDLEs have their own namespace separate
               from FILEHANDLEs.

       ord EXPR
       ord     Returns the numeric (the native 8-bit encoding,
               like ASCII or EBCDIC, or Unicode) value of the
               first character of EXPR.  If EXPR is omitted, uses

               For the reverse, see "chr".  See perlunicode and
               encoding for more about Unicode.

       our EXPR
       our EXPR TYPE
       our EXPR : ATTRS

               An "our" declaration declares a global variable
               that will be visible across its entire lexical
               scope, even across package boundaries.  The pack­
               age in which the variable is entered is determined
               at the point of the declaration, not at the point
               of use.  This means the following behavior holds:

                   package Foo;
                   our $bar;           # declares $Foo::bar for rest of lexical scope
                   $bar = 20;

                   package Bar;
                   print $bar;         # prints 20

               Multiple "our" declarations in the same lexical
               scope are allowed if they are in different pack­
               ages.  If they happened to be in the same package,
               Perl will emit warnings if you have asked for

                   use warnings;
                   package Foo;
                   our $bar;           # declares $Foo::bar for rest of lexical scope
                   $bar = 20;

                   package Bar;
                   our $bar = 30;      # declares $Bar::bar for rest of lexical scope
                   print $bar;         # prints 30

                   our $bar;           # emits warning

               An "our" declaration may also have a list of
               attributes associated with it.

               The exact semantics and interface of TYPE and
               ATTRS are still evolving.  TYPE is currently bound
               to the use of "fields" pragma, and attributes are
               handled using the "attributes" pragma, or starting
               from Perl 5.8.0 also via the "Attribute::Handlers"
               module.  See "Private Variables via my()" in perl­
               sub for details, and fields, attributes, and

               The only currently recognized "our()" attribute is
               "unique" which indicates that a single copy of the
               global is to be used by all interpreters should
               the program happen to be running in a multi-inter­
               preter environment. (The default behaviour would
               be for each interpreter to have its own copy of
               the global.)  Examples:

       pack TEMPLATE,LIST
               Takes a LIST of values and converts it into a
               string using the rules given by the TEMPLATE.  The
               resulting string is the concatenation of the con­
               verted values.  Typically, each converted value
               looks like its machine-level representation.  For
               example, on 32-bit machines a converted integer
               may be represented by a sequence of 4 bytes.

               The TEMPLATE is a sequence of characters that give
               the order and type of values, as follows:

                   a   A string with arbitrary binary data, will be null padded.
                   A   A text (ASCII) string, will be space padded.
                   Z   A null terminated (ASCIZ) string, will be null padded.

                   b   A bit string (ascending bit order inside each byte, like vec()).
                   B   A bit string (descending bit order inside each byte).
                   h   A hex string (low nybble first).
                   H   A hex string (high nybble first).

                   c   A signed char value.
                   C   An unsigned char value.  Only does bytes.  See U for Unicode.

                   s   A signed short value.
                   S   An unsigned short value.
                         (This 'short' is _exactly_ 16 bits, which may differ from
                          what a local C compiler calls 'short'.  If you want
                          native-length shorts, use the '!' suffix.)

                   i   A signed integer value.
                   I   An unsigned integer value.
                         (This 'integer' is _at_least_ 32 bits wide.  Its exact
                          size depends on what a local C compiler calls 'int',
                          and may even be larger than the 'long' described in
                          the next item.)

                   l   A signed long value.
                   L   An unsigned long value.
                         (This 'long' is _exactly_ 32 bits, which may differ from
                          what a local C compiler calls 'long'.  If you want
                          native-length longs, use the '!' suffix.)

                   n   An unsigned short in "network" (big-endian) order.
                   N   An unsigned long in "network" (big-endian) order.
                   v   An unsigned short in "VAX" (little-endian) order.
                   V   An unsigned long in "VAX" (little-endian) order.
                         (These 'shorts' and 'longs' are _exactly_ 16 bits and
                          _exactly_ 32 bits, respectively.)

                   q   A signed quad (64-bit) value.
                         (Long doubles are available only if your system supports long
                          double values _and_ if Perl has been compiled to support those.
                          Causes a fatal error otherwise.)

                   p   A pointer to a null-terminated string.
                   P   A pointer to a structure (fixed-length string).

                   u   A uuencoded string.
                   U   A Unicode character number.  Encodes to UTF-8 internally
                       (or UTF-EBCDIC in EBCDIC platforms).

                   w   A BER compressed integer.  Its bytes represent an unsigned
                       integer in base 128, most significant digit first, with as
                       few digits as possible.  Bit eight (the high bit) is set
                       on each byte except the last.

                   x   A null byte.
                   X   Back up a byte.
                   @   Null fill to absolute position, counted from the start of
                       the innermost ()-group.
                   (   Start of a ()-group.

               The following rules apply:

               *       Each letter may optionally be followed by
                       a number giving a repeat count.  With all
                       types except "a", "A", "Z", "b", "B", "h",
                       "H", "@", "x", "X" and "P" the pack func­
                       tion will gobble up that many values from
                       the LIST.  A "*" for the repeat count
                       means to use however many items are left,
                       except for "@", "x", "X", where it is
                       equivalent to 0, and "u", where it is
                       equivalent to 1 (or 45, what is the same).
                       A numeric repeat count may optionally be
                       enclosed in brackets, as in "pack 'C[80]',

                       One can replace the numeric repeat count
                       by a template enclosed in brackets; then
                       the packed length of this template in
                       bytes is used as a count.  For example,
                       "x[L]" skips a long (it skips the number
                       of bytes in a long); the template "$t
                       X[$t] $t" unpack()s twice what $t unpacks.
                       If the template in brackets contains
                       alignment commands (such as "x![d]"), its
                       packed length is calculated as if the
                       start of the template has the maximal pos­
                       sible alignment.

                       When used with "Z", "*" results in the
                       everything after the first null, and "a"
                       returns data verbatim.  When packing, "a",
                       and "Z" are equivalent.

                       If the value-to-pack is too long, it is
                       truncated.  If too long and an explicit
                       count is provided, "Z" packs only
                       "$count-1" bytes, followed by a null byte.
                       Thus "Z" always packs a trailing null byte
                       under all circumstances.

               *       Likewise, the "b" and "B" fields pack a
                       string that many bits long.  Each byte of
                       the input field of pack() generates 1 bit
                       of the result.  Each result bit is based
                       on the least-significant bit of the corre­
                       sponding input byte, i.e., on
                       "ord($byte)%2".  In particular, bytes "0"
                       and "1" generate bits 0 and 1, as do bytes
                       "\0" and "\1".

                       Starting from the beginning of the input
                       string of pack(), each 8-tuple of bytes is
                       converted to 1 byte of output.  With for­
                       mat "b" the first byte of the 8-tuple
                       determines the least-significant bit of a
                       byte, and with format "B" it determines
                       the most-significant bit of a byte.

                       If the length of the input string is not
                       exactly divisible by 8, the remainder is
                       packed as if the input string were padded
                       by null bytes at the end.  Similarly, dur­
                       ing unpack()ing the "extra" bits are

                       If the input string of pack() is longer
                       than needed, extra bytes are ignored.  A
                       "*" for the repeat count of pack() means
                       to use all the bytes of the input field.
                       On unpack()ing the bits are converted to a
                       string of "0"s and "1"s.

               *       The "h" and "H" fields pack a string that
                       many nybbles (4-bit groups, representable
                       as hexadecimal digits, 0-9a-f) long.

                       Each byte of the input field of pack()
                       generates 4 bits of the result.  For non-
                       alphabetical bytes the result is based on
                       the 4 least-significant bits of the input
                       byte, i.e., on "ord($byte)%16".  In par­
                       output byte, and with format "H" it deter­
                       mines the most-significant nybble.

                       If the length of the input string is not
                       even, it behaves as if padded by a null
                       byte at the end.  Similarly, during
                       unpack()ing the "extra" nybbles are

                       If the input string of pack() is longer
                       than needed, extra bytes are ignored.  A
                       "*" for the repeat count of pack() means
                       to use all the bytes of the input field.
                       On unpack()ing the bits are converted to a
                       string of hexadecimal digits.

               *       The "p" type packs a pointer to a null-
                       terminated string.  You are responsible
                       for ensuring the string is not a temporary
                       value (which can potentially get deallo­
                       cated before you get around to using the
                       packed result).  The "P" type packs a
                       pointer to a structure of the size indi­
                       cated by the length.  A NULL pointer is
                       created if the corresponding value for "p"
                       or "P" is "undef", similarly for unpack().

               *       The "/" template character allows packing
                       and unpacking of strings where the packed
                       structure contains a byte count followed
                       by the string itself.  You write length-

                       The length-item can be any "pack" template
                       letter, and describes how the length value
                       is packed.  The ones likely to be of most
                       use are integer-packing ones like "n" (for
                       Java strings), "w" (for ASN.1 or SNMP) and
                       "N" (for Sun XDR).

                       For "pack", the string-item must, at pre­
                       sent, be "A*", "a*" or "Z*". For "unpack"
                       the length of the string is obtained from
                       the length-item, but if you put in the '*'
                       it will be ignored. For all other codes,
                       "unpack" applies the length value to the
                       next item, which must not have a repeat

                           unpack 'C/a', "\04Gurusamy";        gives 'Guru'
                           unpack 'a3/A* A*', '007 Bond  J ';  gives (' Bond','J')
                           pack 'n/a* w/a*','hello,','world';  gives "\000\006hello,\005world"

                       fix to signify native shorts or longs--as
                       you can see from above for example a bare
                       "l" does mean exactly 32 bits, the native
                       "long" (as seen by the local C compiler)
                       may be larger.  This is an issue mainly in
                       64-bit platforms.  You can see whether
                       using "!" makes any difference by

                               print length(pack("s")), " ", length(pack("s!")), "\n";
                               print length(pack("l")), " ", length(pack("l!")), "\n";

                       "i!" and "I!" also work but only because
                       of completeness; they are identical to "i"
                       and "I".

                       The actual sizes (in bytes) of native
                       shorts, ints, longs, and long longs on the
                       platform where Perl was built are also
                       available via Config:

                              use Config;
                              print $Config{shortsize},    "\n";
                              print $Config{intsize},      "\n";
                              print $Config{longsize},     "\n";
                              print $Config{longlongsize}, "\n";

                       (The $Config{longlongsize} will be unde­
                       fined if your system does not support long

               *       The integer formats "s", "S", "i", "I",
                       "l", "L", "j", and "J" are inherently non-
                       portable between processors and operating
                       systems because they obey the native byte­
                       order and endianness.  For example a
                       4-byte integer 0x12345678 (305419896 deci­
                       mal) would be ordered natively (arranged
                       in and handled by the CPU registers) into
                       bytes as

                               0x12 0x34 0x56 0x78     # big-endian
                               0x78 0x56 0x34 0x12     # little-endian

                       Basically, the Intel and VAX CPUs are lit­
                       tle-endian, while everybody else, for
                       example Motorola m68k/88k, PPC, Sparc, HP
                       PA, Power, and Cray are big-endian.  Alpha
                       and MIPS can be either: Digital/Compaq
                       used/uses them in little-endian mode;
                       SGI/Cray uses them in big-endian mode.

                       The names `big-endian' and `little-endian'
                               print join(" ", map { sprintf "%#02x", $_ }
                                                   unpack("C*",pack("L",0x12345678))), "\n";

                       The byteorder on the platform where Perl
                       was built is also available via Config:

                               use Config;
                               print $Config{byteorder}, "\n";

                       Byteorders '1234' and '12345678' are lit­
                       tle-endian, '4321' and '87654321' are

                       If you want portable packed integers use
                       the formats "n", "N", "v", and "V", their
                       byte endianness and size are known.  See
                       also perlport.

               *       Real numbers (floats and doubles) are in
                       the native machine format only; due to the
                       multiplicity of floating formats around,
                       and the lack of a standard "network" rep­
                       resentation, no facility for interchange
                       has been made.  This means that packed
                       floating point data written on one machine
                       may not be readable on another - even if
                       both use IEEE floating point arithmetic
                       (as the endian-ness of the memory repre­
                       sentation is not part of the IEEE spec).
                       See also perlport.

                       Note that Perl uses doubles internally for
                       all numeric calculation, and converting
                       from double into float and thence back to
                       double again will lose precision (i.e.,
                       "unpack("f", pack("f", $foo)") will not in
                       general equal $foo).

               *       If the pattern begins with a "U", the
                       resulting string will be treated as
                       UTF-8-encoded Unicode. You can force UTF-8
                       encoding on in a string with an initial
                       "U0", and the bytes that follow will be
                       interpreted as Unicode characters. If you
                       don't want this to happen, you can begin
                       your pattern with "C0" (or anything else)
                       to force Perl not to UTF-8 encode your
                       string, and then follow this with a "U*"
                       somewhere in your pattern.

               *       You must yourself do any alignment or
                       padding by inserting for example enough

                           pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )

                       is the string "\0a\0\0bc".

               *       "x" and "X" accept "!" modifier.  In this
                       case they act as alignment commands: they
                       jump forward/back to the closest position
                       aligned at a multiple of "count" bytes.
                       For example, to pack() or unpack() C's
                       "struct {char c; double d; char cc[2]}"
                       one may need to use the template "C x![d]
                       d C[2]"; this assumes that doubles must be
                       aligned on the double's size.

                       For alignment commands "count" of 0 is
                       equivalent to "count" of 1; both result in

               *       A comment in a TEMPLATE starts with "#"
                       and goes to the end of line.  White space
                       may be used to separate pack codes from
                       each other, but a "!" modifier and a
                       repeat count must follow immediately.

               *       If TEMPLATE requires more arguments to
                       pack() than actually given, pack() assumes
                       additional "" arguments.  If TEMPLATE
                       requires less arguments to pack() than
                       actually given, extra arguments are


                   $foo = pack("CCCC",65,66,67,68);
                   # foo eq "ABCD"
                   $foo = pack("C4",65,66,67,68);
                   # same thing
                   $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
                   # same thing with Unicode circled letters

                   $foo = pack("ccxxcc",65,66,67,68);
                   # foo eq "AB\0\0CD"

                   # note: the above examples featuring "C" and "c" are true
                   # only on ASCII and ASCII-derived systems such as ISO Latin 1
                   # and UTF-8.  In EBCDIC the first example would be
                   # $foo = pack("CCCC",193,194,195,196);

                   $foo = pack("s2",1,2);
                   # "\1\0\2\0" on little-endian
                   # "\0\1\0\2" on big-endian
                   $utmp = pack($utmp_template, @utmp1);
                   # a struct utmp (BSDish)

                   @utmp2 = unpack($utmp_template, $utmp);
                   # "@utmp1" eq "@utmp2"

                   sub bintodec {
                       unpack("N", pack("B32", substr("0" x 32 . shift, -32)));

                   $foo = pack('sx2l', 12, 34);
                   # short 12, two zero bytes padding, long 34
                   $bar = pack('s@4l', 12, 34);
                   # short 12, zero fill to position 4, long 34
                   # $foo eq $bar

               The same template may generally also be used in

       package NAMESPACE
       package Declares the compilation unit as being in the
               given namespace.  The scope of the package decla­
               ration is from the declaration itself through the
               end of the enclosing block, file, or eval (the
               same as the "my" operator).  All further unquali­
               fied dynamic identifiers will be in this names­
               pace.  A package statement affects only dynamic
               variables--including those you've used "local"
               on--but not lexical variables, which are created
               with "my".  Typically it would be the first decla­
               ration in a file to be included by the "require"
               or "use" operator.  You can switch into a package
               in more than one place; it merely influences which
               symbol table is used by the compiler for the rest
               of that block.  You can refer to variables and
               filehandles in other packages by prefixing the
               identifier with the package name and a double
               colon:  $Package::Variable.  If the package name
               is null, the "main" package as assumed.  That is,
               $::sail is equivalent to $main::sail (as well as
               to $main'sail, still seen in older code).

               If NAMESPACE is omitted, then there is no current
               package, and all identifiers must be fully quali­
               fied or lexicals.  However, you are strongly
               advised not to make use of this feature. Its use
               can cause unexpected behaviour, even crashing some
               versions of Perl. It is deprecated, and will be
               removed from a future release.

               See "Packages" in perlmod for more information
               about packages, modules, and classes.  See perlsub

               On systems that support a close-on-exec flag on
               files, the flag will be set for the newly opened
               file descriptors as determined by the value of
               $^F.  See "$^F" in perlvar.

       pop ARRAY
       pop     Pops and returns the last value of the array,
               shortening the array by one element.  Has an
               effect similar to


               If there are no elements in the array, returns the
               undefined value (although this may happen at other
               times as well).  If ARRAY is omitted, pops the
               @ARGV array in the main program, and the @_ array
               in subroutines, just like "shift".

       pos SCALAR
       pos     Returns the offset of where the last "m//g" search
               left off for the variable in question ($_ is used
               when the variable is not specified).  May be modi­
               fied to change that offset.  Such modification
               will also influence the "\G" zero-width assertion
               in regular expressions.  See perlre and perlop.

       print FILEHANDLE LIST
       print LIST
       print   Prints a string or a list of strings.  Returns
               true if successful.  FILEHANDLE may be a scalar
               variable name, in which case the variable contains
               the name of or a reference to the filehandle, thus
               introducing one level of indirection.  (NOTE: If
               FILEHANDLE is a variable and the next token is a
               term, it may be misinterpreted as an operator
               unless you interpose a "+" or put parentheses
               around the arguments.)  If FILEHANDLE is omitted,
               prints by default to standard output (or to the
               last selected output channel--see "select").  If
               LIST is also omitted, prints $_ to the currently
               selected output channel.  To set the default out­
               put channel to something other than STDOUT use the
               select operation.  The current value of $, (if
               any) is printed between each LIST item.  The cur­
               rent value of "$\" (if any) is printed after the
               entire LIST has been printed.  Because print takes
               a LIST, anything in the LIST is evaluated in list
               context, and any subroutine that you call will
               have one or more of its expressions evaluated in
               list context.  Also be careful not to follow the
               print keyword with a left parenthesis unless you
               LIST)", except that "$\" (the output record sepa­
               rator) is not appended.  The first argument of the
               list will be interpreted as the "printf" format.
               See "sprintf" for an explanation of the format
               argument. If "use locale" is in effect, the char­
               acter used for the decimal point in formatted real
               numbers is affected by the LC_NUMERIC locale.  See

               Don't fall into the trap of using a "printf" when
               a simple "print" would do.  The "print" is more
               efficient and less error prone.

       prototype FUNCTION
               Returns the prototype of a function as a string
               (or "undef" if the function has no prototype).
               FUNCTION is a reference to, or the name of, the
               function whose prototype you want to retrieve.

               If FUNCTION is a string starting with "CORE::",
               the rest is taken as a name for Perl builtin.  If
               the builtin is not overridable (such as "qw//") or
               its arguments cannot be expressed by a prototype
               (such as "system") returns "undef" because the
               builtin does not really behave like a Perl func­
               tion.  Otherwise, the string describing the equiv­
               alent prototype is returned.

       push ARRAY,LIST
               Treats ARRAY as a stack, and pushes the values of
               LIST onto the end of ARRAY.  The length of ARRAY
               increases by the length of LIST.  Has the same
               effect as

                   for $value (LIST) {
                       $ARRAY[++$#ARRAY] = $value;

               but is more efficient.  Returns the new number of
               elements in the array.

               Generalized quotes.  See "Regexp Quote-Like Opera­
               tors" in perlop.

       quotemeta EXPR
               Returns the value of EXPR with all non-"word"
               is also special-cased as 1 - this has not been
               documented before perl 5.8.0 and is subject to
               change in future versions of perl.  Automatically
               calls "srand" unless "srand" has already been
               called.  See also "srand".

               Apply "int()" to the value returned by "rand()" if
               you want random integers instead of random frac­
               tional numbers.  For example,


               returns a random integer between 0 and 9, inclu­

               (Note: If your rand function consistently returns
               numbers that are too large or too small, then your
               version of Perl was probably compiled with the
               wrong number of RANDBITS.)

               Attempts to read LENGTH characters of data into
               variable SCALAR from the specified FILEHANDLE.
               Returns the number of characters actually read, 0
               at end of file, or undef if there was an error (in
               the latter case $! is also set).  SCALAR will be
               grown or shrunk so that the last character actu­
               ally read is the last character of the scalar
               after the read.

               An OFFSET may be specified to place the read data
               at some place in the string other than the begin­
               ning.  A negative OFFSET specifies placement at
               that many characters counting backwards from the
               end of the string.  A positive OFFSET greater than
               the length of SCALAR results in the string being
               padded to the required size with "\0" bytes before
               the result of the read is appended.

               The call is actually implemented in terms of
               either Perl's or system's fread() call.  To get a
               true read(2) system call, see "sysread".

               Note the characters: depending on the status of
               the filehandle, either (8-bit) bytes or characters
               are read.  By default all filehandles operate on
               bytes, but for example if the filehandle has been
               opened with the ":utf8" I/O layer (see "open", and
               the "open" pragma, open), the I/O will operate on
               UTF-8 encoded Unicode characters, not bytes.  Sim­
               ilarly for the ":encoding" pragma: in that case
               the wrong file.

                   opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
                   @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
                   closedir DIR;

       readline EXPR
               Reads from the filehandle whose typeglob is con­
               tained in EXPR.  In scalar context, each call
               reads and returns the next line, until end-of-file
               is reached, whereupon the subsequent call returns
               undef.  In list context, reads until end-of-file
               is reached and returns a list of lines.  Note that
               the notion of "line" used here is however you may
               have defined it with $/ or $INPUT_RECORD_SEPARA­
               TOR).  See "$/" in perlvar.

               When $/ is set to "undef", when readline() is in
               scalar context (i.e. file slurp mode), and when an
               empty file is read, it returns '' the first time,
               followed by "undef" subsequently.

               This is the internal function implementing the
               "<EXPR>" operator, but you can use it directly.
               The "<EXPR>" operator is discussed in more detail
               in "I/O Operators" in perlop.

                   $line = <STDIN>;
                   $line = readline(*STDIN);           # same thing

               If readline encounters an operating system error,
               $! will be set with the corresponding error mes­
               sage.  It can be helpful to check $! when you are
               reading from filehandles you don't trust, such as
               a tty or a socket.  The following example uses the
               operator form of "readline", and takes the neces­
               sary steps to ensure that "readline" was success­

                   for (;;) {
                       undef $!;
                       unless (defined( $line = <> )) {
                           die $! if $!;
                           last; # reached EOF
                       # ...

       readlink EXPR
               Returns the value of a symbolic link, if symbolic
               links are implemented.  If not, gives a fatal
               detail in "I/O Operators" in perlop.

               Receives a message on a socket.  Attempts to
               receive LENGTH characters of data into variable
               SCALAR from the specified SOCKET filehandle.
               SCALAR will be grown or shrunk to the length actu­
               ally read.  Takes the same flags as the system
               call of the same name.  Returns the address of the
               sender if SOCKET's protocol supports this; returns
               an empty string otherwise.  If there's an error,
               returns the undefined value.  This call is actu­
               ally implemented in terms of recvfrom(2) system
               call.  See "UDP: Message Passing" in perlipc for

               Note the characters: depending on the status of
               the socket, either (8-bit) bytes or characters are
               received.  By default all sockets operate on
               bytes, but for example if the socket has been
               changed using binmode() to operate with the
               ":utf8" I/O layer (see the "open" pragma, open),
               the I/O will operate on UTF-8 encoded Unicode
               characters, not bytes.  Similarly for the ":encod­
               ing" pragma: in that case pretty much any charac­
               ters can be read.

       redo LABEL
       redo    The "redo" command restarts the loop block without
               evaluating the conditional again.  The "continue"
               block, if any, is not executed.  If the LABEL is
               omitted, the command refers to the innermost
               enclosing loop.  This command is normally used by
               programs that want to lie to themselves about what
               was just input:

                   # a simpleminded Pascal comment stripper
                   # (warning: assumes no { or } in strings)
                   LINE: while (<STDIN>) {
                       while (s|({.*}.*){.*}|$1 |) {}
                       s|{.*}| |;
                       if (s|{.*| |) {
                           $front = $_;
                           while (<STDIN>) {
                               if (/}/) {      # end of comment?
                                   redo LINE;

       ref EXPR
       ref     Returns a true value if EXPR is a reference, false
               otherwise.  If EXPR is not specified, $_ will be
               used.  The value returned depends on the type of
               thing the reference is a reference to.  Builtin
               types include:


               If the referenced object has been blessed into a
               package, then that package name is returned
               instead.  You can think of "ref" as a "typeof"

                   if (ref($r) eq "HASH") {
                       print "r is a reference to a hash.\n";
                   unless (ref($r)) {
                       print "r is not a reference at all.\n";
                   if (UNIVERSAL::isa($r, "HASH")) {  # for subclassing
                       print "r is a reference to something that isa hash.\n";

               See also perlref.

       rename OLDNAME,NEWNAME
               Changes the name of a file; an existing file NEW­
               NAME will be clobbered.  Returns true for success,
               false otherwise.

               Behavior of this function varies wildly depending
               on your system implementation.  For example, it
               will usually not work across file system bound­
               aries, even though the system mv command sometimes
               compensates for this.  Other restrictions include
               whether it works on directories, open files, or
               pre-existing files.  Check perlport and either the
               rename(2) manpage or equivalent system documenta­
               tion for details.

       require VERSION
       require EXPR
       require Demands a version of Perl specified by VERSION, or
               demands some semantics specified by EXPR or by $_
               if EXPR is not supplied.

                   require v5.6.1;     # run time version check
                   require 5.6.1;      # ditto
                   require 5.006_001;  # ditto; preferred for backwards compatibility

               Otherwise, demands that a library file be included
               if it hasn't already been included.  The file is
               included via the do-FILE mechanism, which is
               essentially just a variety of "eval".  Has seman­
               tics similar to the following subroutine:

                   sub require {
                       my($filename) = @_;
                       return 1 if $INC{$filename};
                       ITER: {
                           foreach $prefix (@INC) {
                               $realfilename = "$prefix/$filename";
                               if (-f $realfilename) {
                                   $INC{$filename} = $realfilename;
                                   $result = do $realfilename;
                                   last ITER;
                           die "Can't find $filename in \@INC";
                       delete $INC{$filename} if $@ || !$result;
                       die $@ if $@;
                       die "$filename did not return true value" unless $result;
                       return $result;

               Note that the file will not be included twice
               under the same specified name.  The file must
               return true as the last statement to indicate suc­
               cessful execution of any initialization code, so
               it's customary to end such a file with "1;" unless
               you're sure it'll return true otherwise.  But it's
               better just to put the "1;", in case you add more

               If EXPR is a bareword, the require assumes a ".pm"
               extension and replaces "::" with "/" in the file­
               name for you, to make it easy to load standard
               modules.  This form of loading of modules does not
               risk altering your namespace.

               In other words, if you try this:

                       require Foo::Bar;    # a splendid bareword

               The require function will actually look for the
               finding "Foo::Bar" there.  In this case you can

                       eval "require $class";

               Now that you understand how "require" looks for
               files in the case of a bareword argument, there is
               a little extra functionality going on behind the
               scenes.  Before "require" looks for a ".pm" exten­
               sion, it will first look for a filename with a
               ".pmc" extension.  A file with this extension is
               assumed to be Perl bytecode generated by B::Byte­
               code.  If this file is found, and it's modifica­
               tion time is newer than a coinciding ".pm" non-
               compiled file, it will be loaded in place of that
               non-compiled file ending in a ".pm" extension.

               You can also insert hooks into the import facil­
               ity, by putting directly Perl code into the @INC
               array.  There are three forms of hooks: subroutine
               references, array references and blessed objects.

               Subroutine references are the simplest case.  When
               the inclusion system walks through @INC and
               encounters a subroutine, this subroutine gets
               called with two parameters, the first being a ref­
               erence to itself, and the second the name of the
               file to be included (e.g. "Foo/Bar.pm").  The sub­
               routine should return "undef" or a filehandle,
               from which the file to include will be read.  If
               "undef" is returned, "require" will look at the
               remaining elements of @INC.

               If the hook is an array reference, its first ele­
               ment must be a subroutine reference.  This subrou­
               tine is called as above, but the first parameter
               is the array reference.  This enables to pass
               indirectly some arguments to the subroutine.

               In other words, you can write:

                   push @INC, \&my_sub;
                   sub my_sub {
                       my ($coderef, $filename) = @_;  # $coderef is \&my_sub


                   push @INC, [ \&my_sub, $x, $y, ... ];
                   sub my_sub {
                       my ($arrayref, $filename) = @_;

                   sub new { ... }
                   sub Foo::INC {
                       my ($self, $filename) = @_;

                   # In the main program
                   push @INC, new Foo(...);

               Note that these hooks are also permitted to set
               the %INC entry corresponding to the files they
               have loaded. See "%INC" in perlvar.

               For a yet-more-powerful import facility, see "use"
               and perlmod.

       reset EXPR
       reset   Generally used in a "continue" block at the end of
               a loop to clear variables and reset "??" searches
               so that they work again.  The expression is inter­
               preted as a list of single characters (hyphens
               allowed for ranges).  All variables and arrays
               beginning with one of those letters are reset to
               their pristine state.  If the expression is omit­
               ted, one-match searches ("?pattern?") are reset to
               match again.  Resets only variables or searches in
               the current package.  Always returns 1.  Examples:

                   reset 'X';          # reset all X variables
                   reset 'a-z';        # reset lower case variables
                   reset;              # just reset ?one-time? searches

               Resetting "A-Z" is not recommended because you'll
               wipe out your @ARGV and @INC arrays and your %ENV
               hash.  Resets only package variables--lexical
               variables are unaffected, but they clean them­
               selves up on scope exit anyway, so you'll probably
               want to use them instead.  See "my".

       return EXPR
       return  Returns from a subroutine, "eval", or "do FILE"
               with the value given in EXPR.  Evaluation of EXPR
               may be in list, scalar, or void context, depending
               on how the return value will be used, and the con­
               text may vary from one execution to the next (see
               "wantarray").  If no EXPR is given, returns an
               empty list in list context, the undefined value in
               scalar context, and (of course) nothing at all in
               a void context.

               (Note that in the absence of an explicit "return",
               a subroutine, eval, or do FILE will automatically

               This operator is also handy for inverting a hash,
               although there are some caveats.  If a value is
               duplicated in the original hash, only one of those
               can be represented as a key in the inverted hash.
               Also, this has to unwind one hash and build a
               whole new one, which may take some time on a large
               hash, such as from a DBM file.

                   %by_name = reverse %by_address;     # Invert the hash

       rewinddir DIRHANDLE
               Sets the current position to the beginning of the
               directory for the "readdir" routine on DIRHANDLE.

       rindex STR,SUBSTR
               Works just like index() except that it returns the
               position of the LAST occurrence of SUBSTR in STR.
               If POSITION is specified, returns the last occur­
               rence at or before that position.

       rmdir FILENAME
       rmdir   Deletes the directory specified by FILENAME if
               that directory is empty.  If it succeeds it
               returns true, otherwise it returns false and sets
               $! (errno).  If FILENAME is omitted, uses $_.

       s///    The substitution operator.  See perlop.

       scalar EXPR
               Forces EXPR to be interpreted in scalar context
               and returns the value of EXPR.

                   @counts = ( scalar @a, scalar @b, scalar @c );

               There is no equivalent operator to force an
               expression to be interpolated in list context
               because in practice, this is never needed.  If you
               really wanted to do so, however, you could use the
               construction "@{[ (some expression) ]}", but usu­
               ally a simple "(some expression)" suffices.

               Because "scalar" is unary operator, if you acci­
               dentally use for EXPR a parenthesized list, this
               behaves as a scalar comma expression, evaluating
               all but the last element in void context and
               returning the final element evaluated in scalar
               context.  This is seldom what you want.

               The following single statement:

               values for WHENCE are 0 to set the new position in
               bytes to POSITION, 1 to set it to the current
               position plus POSITION, and 2 to set it to EOF
               plus POSITION (typically negative).  For WHENCE
               you may use the constants "SEEK_SET", "SEEK_CUR",
               and "SEEK_END" (start of the file, current posi­
               tion, end of the file) from the Fcntl module.
               Returns 1 upon success, 0 otherwise.

               Note the in bytes: even if the filehandle has been
               set to operate on characters (for example by using
               the ":utf8" open layer), tell() will return byte
               offsets, not character offsets (because implement­
               ing that would render seek() and tell() rather

               If you want to position file for "sysread" or
               "syswrite", don't use "seek"--buffering makes its
               effect on the file's system position unpredictable
               and non-portable.  Use "sysseek" instead.

               Due to the rules and rigors of ANSI C, on some
               systems you have to do a seek whenever you switch
               between reading and writing.  Amongst other
               things, this may have the effect of calling
               stdio's clearerr(3).  A WHENCE of 1 ("SEEK_CUR")
               is useful for not moving the file position:


               This is also useful for applications emulating
               "tail -f".  Once you hit EOF on your read, and
               then sleep for a while, you might have to stick in
               a seek() to reset things.  The "seek" doesn't
               change the current position, but it does clear the
               end-of-file condition on the handle, so that the
               next "<FILE>" makes Perl try again to read some­
               thing.  We hope.

               If that doesn't work (some IO implementations are
               particularly cantankerous), then you may need
               something more like this:

                   for (;;) {
                       for ($curpos = tell(FILE); $_ = <FILE>;
                            $curpos = tell(FILE)) {
                           # search for some stuff and put it into files
                       seek(FILE, $curpos, 0);

               this output channel.  For example, if you have to
               set the top of form format for more than one out­
               put channel, you might do the following:

                   $^ = 'report1_top';
                   $^ = 'report2_top';

               FILEHANDLE may be an expression whose value gives
               the name of the actual filehandle.  Thus:

                   $oldfh = select(STDERR); $| = 1; select($oldfh);

               Some programmers may prefer to think of filehan­
               dles as objects with methods, preferring to write
               the last example as:

                   use IO::Handle;

               This calls the select(2) system call with the bit
               masks specified, which can be constructed using
               "fileno" and "vec", along these lines:

                   $rin = $win = $ein = '';
                   vec($rin,fileno(STDIN),1) = 1;
                   vec($win,fileno(STDOUT),1) = 1;
                   $ein = $rin | $win;

               If you want to select on many filehandles you
               might wish to write a subroutine:

                   sub fhbits {
                       my(@fhlist) = split(' ',$_[0]);
                       for (@fhlist) {
                           vec($bits,fileno($_),1) = 1;
                   $rin = fhbits('STDIN TTY SOCK');

               The usual idiom is:

                   ($nfound,$timeleft) =
                     select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

               or to block until something becomes ready just do


                   select(undef, undef, undef, 0.25);

               Note that whether "select" gets restarted after
               signals (say, SIGALRM) is implementation-depen­

               WARNING: One should not attempt to mix buffered
               I/O (like "read" or <FH>) with "select", except as
               permitted by POSIX, and even then only on POSIX
               systems.  You have to use "sysread" instead.

       semctl ID,SEMNUM,CMD,ARG
               Calls the System V IPC function "semctl".  You'll
               probably have to say

                   use IPC::SysV;

               first to get the correct constant definitions.  If
               CMD is IPC_STAT or GETALL, then ARG must be a
               variable which will hold the returned semid_ds
               structure or semaphore value array.  Returns like
               "ioctl": the undefined value for error, ""0 but
               true"" for zero, or the actual return value other­
               wise.  The ARG must consist of a vector of native
               short integers, which may be created with
               "pack("s!",(0)x$nsem)".  See also "SysV IPC" in
               perlipc, "IPC::SysV", "IPC::Semaphore" documenta­

       semget KEY,NSEMS,FLAGS
               Calls the System V IPC function semget.  Returns
               the semaphore id, or the undefined value if there
               is an error.  See also "SysV IPC" in perlipc,
               "IPC::SysV", "IPC::SysV::Semaphore" documentation.

       semop KEY,OPSTRING
               Calls the System V IPC function semop to perform
               semaphore operations such as signalling and wait­
               ing.  OPSTRING must be a packed array of semop
               structures.  Each semop structure can be generated
               with "pack("s!3", $semnum, $semop, $semflag)".
               The number of semaphore operations is implied by
               the length of OPSTRING.  Returns true if success­
               ful, or false if there is an error.  As an exam­
               ple, the following code waits on semaphore $semnum
               of semaphore id $semid:

                   $semop = pack("s!3", $semnum, -1, 0);
                   die "Semaphore trouble: $!\n" unless semop($semid, $semop);

               mented.  See "UDP: Message Passing" in perlipc for

               Note the characters: depending on the status of
               the socket, either (8-bit) bytes or characters are
               sent.  By default all sockets operate on bytes,
               but for example if the socket has been changed
               using binmode() to operate with the ":utf8" I/O
               layer (see "open", or the "open" pragma, open),
               the I/O will operate on UTF-8 encoded Unicode
               characters, not bytes.  Similarly for the ":encod­
               ing" pragma: in that case pretty much any charac­
               ters can be sent.

       setpgrp PID,PGRP
               Sets the current process group for the specified
               PID, 0 for the current process.  Will produce a
               fatal error if used on a machine that doesn't
               implement POSIX setpgid(2) or BSD setpgrp(2).  If
               the arguments are omitted, it defaults to "0,0".
               Note that the BSD 4.2 version of "setpgrp" does
               not accept any arguments, so only "setpgrp(0,0)"
               is portable.  See also "POSIX::setsid()".

       setpriority WHICH,WHO,PRIORITY
               Sets the current priority for a process, a process
               group, or a user.  (See setpriority(2).)  Will
               produce a fatal error if used on a machine that
               doesn't implement setpriority(2).

               Sets the socket option requested.  Returns unde­
               fined if there is an error.  OPTVAL may be speci­
               fied as "undef" if you don't want to pass an argu­

       shift ARRAY
       shift   Shifts the first value of the array off and
               returns it, shortening the array by 1 and moving
               everything down.  If there are no elements in the
               array, returns the undefined value.  If ARRAY is
               omitted, shifts the @_ array within the lexical
               scope of subroutines and formats, and the @ARGV
               array at file scopes or within the lexical scopes
               established by the "eval ''", "BEGIN {}", "INIT
               {}", "CHECK {}", and "END {}" constructs.

               See also "unshift", "push", and "pop".  "shift"
               and "unshift" do the same thing to the left end of
               an array that "pop" and "push" do to the right

       shmget KEY,SIZE,FLAGS
               Calls the System V IPC function shmget.  Returns
               the shared memory segment id, or the undefined
               value if there is an error.  See also "SysV IPC"
               in perlipc and "IPC::SysV" documentation.

       shmread ID,VAR,POS,SIZE
       shmwrite ID,STRING,POS,SIZE
               Reads or writes the System V shared memory segment
               ID starting at position POS for size SIZE by
               attaching to it, copying in/out, and detaching
               from it.  When reading, VAR must be a variable
               that will hold the data read.  When writing, if
               STRING is too long, only SIZE bytes are used; if
               STRING is too short, nulls are written to fill out
               SIZE bytes.  Return true if successful, or false
               if there is an error.  shmread() taints the vari­
               able. See also "SysV IPC" in perlipc, "IPC::SysV"
               documentation, and the "IPC::Shareable" module
               from CPAN.

       shutdown SOCKET,HOW
               Shuts down a socket connection in the manner indi­
               cated by HOW, which has the same interpretation as
               in the system call of the same name.

                   shutdown(SOCKET, 0);    # I/we have stopped reading data
                   shutdown(SOCKET, 1);    # I/we have stopped writing data
                   shutdown(SOCKET, 2);    # I/we have stopped using this socket

               This is useful with sockets when you want to tell
               the other side you're done writing but not done
               reading, or vice versa.  It's also a more insis­
               tent form of close because it also disables the
               file descriptor in any forked copies in other pro­

       sin EXPR
       sin     Returns the sine of EXPR (expressed in radians).
               If EXPR is omitted, returns sine of $_.

               For the inverse sine operation, you may use the
               "Math::Trig::asin" function, or use this relation:

                   sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

       sleep EXPR
       sleep   Causes the script to sleep for EXPR seconds, or
               forever if no EXPR.  May be interrupted if the
               process receives a signal such as "SIGALRM".
               Returns the number of seconds actually slept.  You
               probably cannot mix "alarm" and "sleep" calls,
               see "select" above.  The Time::HiRes module (from
               CPAN, and starting from Perl 5.8 part of the stan­
               dard distribution) may also help.

               See also the POSIX module's "pause" function.

               Opens a socket of the specified kind and attaches
               it to filehandle SOCKET.  DOMAIN, TYPE, and PROTO­
               COL are specified the same as for the system call
               of the same name.  You should "use Socket" first
               to get the proper definitions imported.  See the
               examples in "Sockets: Client/Server Communication"
               in perlipc.

               On systems that support a close-on-exec flag on
               files, the flag will be set for the newly opened
               file descriptor, as determined by the value of
               $^F.  See "$^F" in perlvar.

               Creates an unnamed pair of sockets in the speci­
               fied domain, of the specified type.  DOMAIN, TYPE,
               and PROTOCOL are specified the same as for the
               system call of the same name.  If unimplemented,
               yields a fatal error.  Returns true if successful.

               On systems that support a close-on-exec flag on
               files, the flag will be set for the newly opened
               file descriptors, as determined by the value of
               $^F.  See "$^F" in perlvar.

               Some systems defined "pipe" in terms of "socket­
               pair", in which a call to "pipe(Rdr, Wtr)" is

                   use Socket;
                   socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
                   shutdown(Rdr, 1);        # no more writing for reader
                   shutdown(Wtr, 0);        # no more reading for writer

               See perlipc for an example of socketpair use.
               Perl 5.8 and later will emulate socketpair using
               IP sockets to localhost if your system implements
               sockets but not socketpair.

       sort SUBNAME LIST
       sort BLOCK LIST
       sort LIST
               In list context, this sorts the LIST and returns
               the sorted list value.  In scalar context, the
               behaviour of "sort()" is undefined.
               If the subroutine's prototype is "($$)", the ele­
               ments to be compared are passed by reference in
               @_, as for a normal subroutine.  This is slower
               than unprototyped subroutines, where the elements
               to be compared are passed into the subroutine as
               the package global variables $a and $b (see exam­
               ple below).  Note that in the latter case, it is
               usually counter-productive to declare $a and $b as

               In either case, the subroutine may not be recur­
               sive.  The values to be compared are always passed
               by reference, so don't modify them.

               You also cannot exit out of the sort block or sub­
               routine using any of the loop control operators
               described in perlsyn or with "goto".

               When "use locale" is in effect, "sort LIST" sorts
               LIST according to the current collation locale.
               See perllocale.

               Perl 5.6 and earlier used a quicksort algorithm to
               implement sort.  That algorithm was not stable,
               and could go quadratic.  (A stable sort preserves
               the input order of elements that compare equal.
               Although quicksort's run time is O(NlogN) when
               averaged over all arrays of length N, the time can
               be O(N**2), quadratic behavior, for some inputs.)
               In 5.7, the quicksort implementation was replaced
               with a stable mergesort algorithm whose worst case
               behavior is O(NlogN).  But benchmarks indicated
               that for some inputs, on some platforms, the orig­
               inal quicksort was faster.  5.8 has a sort pragma
               for limited control of the sort.  Its rather blunt
               control of the underlying algorithm may not per­
               sist into future perls, but the ability to charac­
               terize the input or output in implementation inde­
               pendent ways quite probably will.  See sort.


                   # sort lexically
                   @articles = sort @files;

                   # same thing, but with explicit sort routine
                   @articles = sort {$a cmp $b} @files;

                   # now case-insensitively
                   @articles = sort {uc($a) cmp uc($b)} @files;

                   # same thing in reversed order
                       $age{$a} <=> $age{$b};  # presuming numeric
                   @sortedclass = sort byage @class;

                   sub backwards { $b cmp $a }
                   @harry  = qw(dog cat x Cain Abel);
                   @george = qw(gone chased yz Punished Axed);
                   print sort @harry;
                           # prints AbelCaincatdogx
                   print sort backwards @harry;
                           # prints xdogcatCainAbel
                   print sort @george, 'to', @harry;
                           # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

                   # inefficiently sort by descending numeric compare using
                   # the first integer after the first = sign, or the
                   # whole record case-insensitively otherwise

                   @new = sort {
                       ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                                   uc($a)  cmp  uc($b)
                   } @old;

                   # same thing, but much more efficiently;
                   # we'll build auxiliary indices instead
                   # for speed
                   @nums = @caps = ();
                   for (@old) {
                       push @nums, /=(\d+)/;
                       push @caps, uc($_);

                   @new = @old[ sort {
                                       $nums[$b] <=> $nums[$a]
                                       $caps[$a] cmp $caps[$b]
                                      } 0..$#old

                   # same thing, but without any temps
                   @new = map { $_->[0] }
                          sort { $b->[1] <=> $a->[1]
                                 $a->[2] cmp $b->[2]
                          } map { [$_, /=(\d+)/, uc($_)] } @old;

                   # using a prototype allows you to use any comparison subroutine
                   # as a sort subroutine (including other package's subroutines)
                   package other;
                   sub backwards ($$) { $_[1] cmp $_[0]; }     # $a and $b are not set here


                   @articles = sort {$b <=> $a} @files;

               then $a and $b are $main::a and $main::b (or $::a
               and $::b), but if you're in the "FooPack" package,
               it's the same as typing

                   @articles = sort {$FooPack::b <=> $FooPack::a} @files;

               The comparison function is required to behave.  If
               it returns inconsistent results (sometimes saying
               $x[1] is less than $x[2] and sometimes saying the
               opposite, for example) the results are not

               Because "<=>" returns "undef" when either operand
               is "NaN" (not-a-number), and because "sort" will
               trigger a fatal error unless the result of a com­
               parison is defined, when sorting with a comparison
               function like "$a <=> $b", be careful about lists
               that might contain a "NaN".  The following example
               takes advantage of the fact that "NaN != NaN" to
               eliminate any "NaN"s from the input.

                   @result = sort { $a <=> $b } grep { $_ == $_ } @input;

       splice ARRAY,OFFSET
       splice ARRAY
               Removes the elements designated by OFFSET and
               LENGTH from an array, and replaces them with the
               elements of LIST, if any.  In list context,
               returns the elements removed from the array.  In
               scalar context, returns the last element removed,
               or "undef" if no elements are removed.  The array
               grows or shrinks as necessary.  If OFFSET is nega­
               tive then it starts that far from the end of the
               array.  If LENGTH is omitted, removes everything
               from OFFSET onward.  If LENGTH is negative,
               removes the elements from OFFSET onward except for
               -LENGTH elements at the end of the array.  If both
               OFFSET and LENGTH are omitted, removes everything.
               If OFFSET is past the end of the array, perl
               issues a warning, and splices at the end of the

               The following equivalences hold (assuming "$[ == 0
               and $#a >= $i" )

                   push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
                       return 1;
                   if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

       split /PATTERN/,EXPR,LIMIT
       split /PATTERN/,EXPR
       split /PATTERN/
       split   Splits a string into a list of strings and returns
               that list.  By default, empty leading fields are
               preserved, and empty trailing ones are deleted.

               In scalar context, returns the number of fields
               found and splits into the @_ array.  Use of split
               in scalar context is deprecated, however, because
               it clobbers your subroutine arguments.

               If EXPR is omitted, splits the $_ string.  If PAT­
               TERN is also omitted, splits on whitespace (after
               skipping any leading whitespace).  Anything match­
               ing PATTERN is taken to be a delimiter separating
               the fields.  (Note that the delimiter may be
               longer than one character.)

               If LIMIT is specified and positive, it represents
               the maximum number of fields the EXPR will be
               split into, though the actual number of fields
               returned depends on the number of times PATTERN
               matches within EXPR.  If LIMIT is unspecified or
               zero, trailing null fields are stripped (which
               potential users of "pop" would do well to remem­
               ber).  If LIMIT is negative, it is treated as if
               an arbitrarily large LIMIT had been specified.
               Note that splitting an EXPR that evaluates to the
               empty string always returns the empty list,
               regardless of the LIMIT specified.

               A pattern matching the null string (not to be con­
               fused with a null pattern "//", which is just one
               member of the set of patterns matching a null
               string) will split the value of EXPR into separate
               characters at each point it matches that way.  For

                   print join(':', split(/ */, 'hi there'));

               produces the output 'h:i:t:h:e:r:e'.

               Using the empty pattern "//" specifically matches
               the null string, and is not be confused with the
               use of "//" to mean "the last successful pattern
                   ($login, $passwd, $remainder) = split(/:/, $_, 3);

               When assigning to a list, if LIMIT is omitted, or
               zero, Perl supplies a LIMIT one larger than the
               number of variables in the list, to avoid unneces­
               sary work.  For the list above LIMIT would have
               been 4 by default.  In time critical applications
               it behooves you not to split into more fields than
               you really need.

               If the PATTERN contains parentheses, additional
               list elements are created from each matching sub­
               string in the delimiter.

                   split(/([,-])/, "1-10,20", 3);

               produces the list value

                   (1, '-', 10, ',', 20)

               If you had the entire header of a normal Unix
               email message in $header, you could split it up
               into fields and their values this way:

                   $header =~ s/\n\s+/ /g;  # fix continuation lines
                   %hdrs   =  (UNIX_FROM => split /^(\S*?):\s*/m, $header);

               The pattern "/PATTERN/" may be replaced with an
               expression to specify patterns that vary at run­
               time.  (To do runtime compilation only once, use

               As a special case, specifying a PATTERN of space
               (' ') will split on white space just as "split"
               with no arguments does.  Thus, "split(' ')" can be
               used to emulate awk's default behavior, whereas
               "split(/ /)" will give you as many null initial
               fields as there are leading spaces.  A "split" on
               "/\s+/" is like a "split(' ')" except that any
               leading whitespace produces a null first field.  A
               "split" with no arguments really does a
               "split(' ', $_)" internally.

               A PATTERN of "/^/" is treated as if it were
               "/^/m", since it isn't much use otherwise.


                   open(PASSWD, '/etc/passwd');
                   while (<PASSWD>) {
                       ($login, $passwd, $uid, $gid,

               See below for more details and see sprintf(3) or
               printf(3) on your system for an explanation of the
               general principles.

               For example:

                       # Format number with up to 8 leading zeroes
                       $result = sprintf("%08d", $number);

                       # Round number to 3 digits after decimal point
                       $rounded = sprintf("%.3f", $number);

               Perl does its own "sprintf" formatting--it emu­
               lates the C function "sprintf", but it doesn't use
               it (except for floating-point numbers, and even
               then only the standard modifiers are allowed).  As
               a result, any non-standard extensions in your
               local "sprintf" are not available from Perl.

               Unlike "printf", "sprintf" does not do what you
               probably mean when you pass it an array as your
               first argument. The array is given scalar context,
               and instead of using the 0th element of the array
               as the format, Perl will use the count of elements
               in the array as the format, which is almost never

               Perl's "sprintf" permits the following univer­
               sally-known conversions:

                  %%   a percent sign
                  %c   a character with the given number
                  %s   a string
                  %d   a signed integer, in decimal
                  %u   an unsigned integer, in decimal
                  %o   an unsigned integer, in octal
                  %x   an unsigned integer, in hexadecimal
                  %e   a floating-point number, in scientific notation
                  %f   a floating-point number, in fixed decimal notation
                  %g   a floating-point number, in %e or %f notation

               In addition, Perl permits the following widely-
               supported conversions:

                  %X   like %x, but using upper-case letters
                  %E   like %e, but using an upper-case "E"
                  %G   like %g, but with an upper-case "E" (if applicable)
                  %b   an unsigned integer, in binary
                  %p   a pointer (outputs the Perl value's address in hexadecimal)
                  %n   special: *stores* the number of characters output so far
                       into the next variable in the parameter list

               scientific notation produced by %e, %E, %g and %G
               for numbers with the modulus of the exponent less
               than 100 is system-dependent: it may be three or
               less (zero-padded as necessary).  In other words,
               1.23 times ten to the 99th may be either "1.23e99"
               or "1.23e099".

               Between the "%" and the format letter, you may
               specify a number of additional attributes control­
               ling the interpretation of the format.  In order,
               these are:

               format parameter index
                   An explicit format parameter index, such as
                   "2$". By default sprintf will format the next
                   unused argument in the list, but this allows
                   you to take the arguments out of order. Eg:

                     printf '%2$d %1$d', 12, 34;      # prints "34 12"
                     printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

                   one or more of:
                      space   prefix positive number with a space
                      +       prefix positive number with a plus
                      -       left-justify within the field
                      0       use zeros, not spaces, to right-
                      #       prefix non-zero octal with "0",
                   non-zero hex with "0x",
                              non-zero binary with "0b"

                   For example:

                     printf '<% d>', 12;   # prints "< 12>"
                     printf '<%+d>', 12;   # prints "<+12>"
                     printf '<%6s>', 12;   # prints "<    12>"
                     printf '<%-6s>', 12;  # prints "<12    >"
                     printf '<%06s>', 12;  # prints "<000012>"
                     printf '<%#x>', 12;   # prints "<0xc>"

               vector flag
                   The vector flag "v", optionally specifying the
                   join string to use.  This flag tells perl to
                   interpret the supplied string as a vector of
                   integers, one for each character in the
                   string, separated by a given string (a dot "."
                   by default). This can be useful for displaying
                   ordinal values of characters in arbitrary

               (minimum) width
                   Arguments are usually formatted to be only as
                   wide as required to display the given value.
                   You can override the width by putting a number
                   here, or get the width from the next argument
                   (with "*") or from a specified argument (with
                   eg "*2$"):

                     printf '<%s>', "a";       # prints "<a>"
                     printf '<%6s>', "a";      # prints "<     a>"
                     printf '<%*s>', 6, "a";   # prints "<     a>"
                     printf '<%*2$s>', "a", 6; # prints "<     a>"
                     printf '<%2s>', "long";   # prints "<long>" (does not truncate)

                   If a field width obtained through "*" is nega­
                   tive, it has the same effect as the "-" flag:

               precision, or maximum width
                   You can specify a precision (for numeric con­
                   versions) or a maximum width (for string con­
                   versions) by specifying a "." followed by a
                   number.  For floating point formats, with the
                   exception of 'g' and 'G', this specifies the
                   number of decimal places to show (the default
                   being 6), eg:

                     # these examples are subject to system-specific variation
                     printf '<%f>', 1;    # prints "<1.000000>"
                     printf '<%.1f>', 1;  # prints "<1.0>"
                     printf '<%.0f>', 1;  # prints "<1>"
                     printf '<%e>', 10;   # prints "<1.000000e+01>"
                     printf '<%.1e>', 10; # prints "<1.0e+01>"

                   For 'g' and 'G', this specifies the maximum
                   number of digits to show, including prior to
                   the decimal point as well as after it, eg:

                     # these examples are subject to system-specific variation
                     printf '<%g>', 1;        # prints "<1>"
                     printf '<%.10g>', 1;     # prints "<1>"
                     printf '<%g>', 100;      # prints "<100>"
                     printf '<%.1g>', 100;    # prints "<1e+02>"
                     printf '<%.2g>', 100.01; # prints "<1e+02>"
                     printf '<%.5g>', 100.01; # prints "<100.01>"
                     printf '<%.4g>', 100.01; # prints "<100>"

                   For integer conversions, specifying a preci­
                   sion implies that the output of the number
                   itself should be zero-padded to this width:

                     printf '<%.6x>', 1;      # prints "<000001>"
                     printf '<%.*x>', 6, 1;    # prints "<000001>"

                   You cannot currently get the precision from a
                   specified number, but it is intended that this
                   will be possible in the future using eg

                     printf '<%.*2$x>', 1, 6;   # INVALID, but in future will print "<000001>"

                   For numeric conversions, you can specify the
                   size to interpret the number as using "l",
                   "h", "V", "q", "L", or "ll". For integer con­
                   versions ("d u o x X b i D U O"), numbers are
                   usually assumed to be whatever the default
                   integer size is on your platform (usually 32
                   or 64 bits), but you can override this to use
                   instead one of the standard C types, as sup­
                   ported by the compiler used to build Perl:

                      l           interpret integer as C type "long" or "unsigned long"
                      h           interpret integer as C type "short" or "unsigned short"
                      q, L or ll  interpret integer as C type "long long", "unsigned long long".
                                  or "quads" (typically 64-bit integers)

                   The last will produce errors if Perl does not
                   understand "quads" in your installation. (This
                   requires that either the platform natively
                   supports quads or Perl was specifically com­
                   piled to support quads.) You can find out
                   whether your Perl supports quads via Config:

                           use Config;
                           ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
                                   print "quads\n";

                   For floating point conversions ("e f g E F
                   G"), numbers are usually assumed to be the
                   default floating point size on your platform
                   (double or long double), but you can force
                   'long double' with "q", "L", or "ll" if your
                   platform supports them. You can find out
                   whether your Perl supports long doubles via

                           use Config;
                           $Config{d_longdbl} eq 'define' && print "long doubles\n";

                   You can find out whether Perl considers 'long
                   double' to be the default floating point size
                   to use on your platform via Config:

                   for a Perl integer (or floating-point num­
                   ber)', which is already the default for Perl

               order of arguments
                   Normally, sprintf takes the next unused argu­
                   ment as the value to format for each format
                   specification. If the format specification
                   uses "*" to require additional arguments,
                   these are consumed from the argument list in
                   the order in which they appear in the format
                   specification before the value to format.
                   Where an argument is specified using an
                   explicit index, this does not affect the nor­
                   mal order for the arguments (even when the
                   explicitly specified index would have been the
                   next argument in any case).


                     printf '<%*.*s>', $a, $b, $c;

                   would use $a for the width, $b for the preci­
                   sion and $c as the value to format, while:

                     print '<%*1$.*s>', $a, $b;

                   would use $a for the width and the precision,
                   and $b as the value to format.

                   Here are some more examples - beware that when
                   using an explicit index, the "$" may need to
                   be escaped:

                     printf "%2\$d %d\n",    12, 34;               # will print "34 12\n"
                     printf "%2\$d %d %d\n", 12, 34;               # will print "34 12 34\n"
                     printf "%3\$d %d %d\n", 12, 34, 56;           # will print "56 12 34\n"
                     printf "%2\$*3\$d %d\n", 12, 34, 3;           # will print " 34 12\n"

               If "use locale" is in effect, the character used
               for the decimal point in formatted real numbers is
               affected by the LC_NUMERIC locale.  See perllo­

       sqrt EXPR
       sqrt    Return the square root of EXPR.  If EXPR is omit­
               ted, returns square root of $_.  Only works on
               non-negative operands, unless you've loaded the
               standard Math::Complex module.

                   use Math::Complex;
                   print sqrt(-2);    # prints 1.4142135623731i

               Most programs won't even call srand() at all,
               except those that need a cryptographically-strong
               starting point rather than the generally accept­
               able default, which is based on time of day, pro­
               cess ID, and memory allocation, or the /dev/uran­
               dom device, if available.

               You can call srand($seed) with the same $seed to
               reproduce the same sequence from rand(), but this
               is usually reserved for generating predictable
               results for testing or debugging.  Otherwise,
               don't call srand() more than once in your program.

               Do not call srand() (i.e. without an argument)
               more than once in a script.  The internal state of
               the random number generator should contain more
               entropy than can be provided by any seed, so call­
               ing srand() again actually loses randomness.

               Most implementations of "srand" take an integer
               and will silently truncate decimal numbers.  This
               means "srand(42)" will usually produce the same
               results as "srand(42.1)".  To be safe, always pass
               "srand" an integer.

               In versions of Perl prior to 5.004 the default
               seed was just the current "time".  This isn't a
               particularly good seed, so many old programs sup­
               ply their own seed value (often "time ^ $$" or
               "time ^ ($$ + ($$ << 15))"), but that isn't neces­
               sary any more.

               Note that you need something much more random than
               the default seed for cryptographic purposes.
               Checksumming the compressed output of one or more
               rapidly changing operating system status programs
               is the usual method.  For example:

                   srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

               If you're particularly concerned with this, see
               the "Math::TrulyRandom" module in CPAN.

               Frequently called programs (like CGI scripts) that
               simply use

                   time ^ $$

               for a seed can fall prey to the mathematical prop­
               erty that

                          = stat($filename);

               Not all fields are supported on all filesystem
               types.  Here are the meaning of the fields:

                 0 dev      device number of filesystem
                 1 ino      inode number
                 2 mode     file mode  (type and permissions)
                 3 nlink    number of (hard) links to the file
                 4 uid      numeric user ID of file's owner
                 5 gid      numeric group ID of file's owner
                 6 rdev     the device identifier (special files only)
                 7 size     total size of file, in bytes
                 8 atime    last access time in seconds since the epoch
                 9 mtime    last modify time in seconds since the epoch
                10 ctime    inode change time in seconds since the epoch (*)
                11 blksize  preferred block size for file system I/O
                12 blocks   actual number of blocks allocated

               (The epoch was at 00:00 January 1, 1970 GMT.)

               (*) The ctime field is non-portable, in particular
               you cannot expect it to be a "creation time", see
               "Files and Filesystems" in perlport for details.

               If stat is passed the special filehandle consist­
               ing of an underline, no stat is done, but the cur­
               rent contents of the stat structure from the last
               stat or filetest are returned.  Example:

                   if (-x $file && (($d) = stat(_)) && $d < 0) {
                       print "$file is executable NFS file\n";

               (This works on machines only for which the device
               number is negative under NFS.)

               Because the mode contains both the file type and
               its permissions, you should mask off the file type
               portion and (s)printf using a "%o" if you want to
               see the real permissions.

                   $mode = (stat($filename))[2];
                   printf "Permissions are %04o\n", $mode & 07777;

               In scalar context, "stat" returns a boolean value
               indicating success or failure, and, if successful,
               sets the information associated with the special
               filehandle "_".

               The File::stat module provides a convenient, by-
               name access mechanism:
                   $user_rwx      = ($mode & S_IRWXU) >> 6;
                   $group_read    = ($mode & S_IRGRP) >> 3;
                   $other_execute =  $mode & S_IXOTH;

                   printf "Permissions are %04o\n", S_IMODE($mode), "\n";

                   $is_setuid     =  $mode & S_ISUID;
                   $is_setgid     =  S_ISDIR($mode);

               You could write the last two using the "-u" and
               "-d" operators.  The commonly available S_IF* con­
               stants are

                   # Permissions: read, write, execute, for user, group, others.

                   S_IRWXU S_IRUSR S_IWUSR S_IXUSR
                   S_IRWXG S_IRGRP S_IWGRP S_IXGRP
                   S_IRWXO S_IROTH S_IWOTH S_IXOTH

                   # Setuid/Setgid/Stickiness/SaveText.
                   # Note that the exact meaning of these is system dependent.

                   S_ISUID S_ISGID S_ISVTX S_ISTXT

                   # File types.  Not necessarily all are available on your system.


                   # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.

                   S_IREAD S_IWRITE S_IEXEC

               and the S_IF* functions are

                   S_IMODE($mode)      the part of $mode containing the permission bits
                                       and the setuid/setgid/sticky bits

                   S_IFMT($mode)       the part of $mode containing the file type
                                       which can be bit-anded with e.g. S_IFREG
                                       or with the following functions

                   # The operators -f, -d, -l, -b, -c, -p, and -s.

                   S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
                   S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

                   # No direct -X operator counterpart, but for the first one
                   # the -g operator is often equivalent.  The ENFMT stands for
                   # record flocking enforcement, a platform-dependent feature.

                   S_ISENFMT($mode) S_ISWHT($mode)

               cies in the string to be searched--you probably
               want to compare run times with and without it to
               see which runs faster.  Those loops which scan for
               many short constant strings (including the con­
               stant parts of more complex patterns) will benefit
               most.  You may have only one "study" active at a
               time--if you study a different scalar the first is
               "unstudied".  (The way "study" works is this: a
               linked list of every character in the string to be
               searched is made, so we know, for example, where
               all the 'k' characters are.  From each search
               string, the rarest character is selected, based on
               some static frequency tables constructed from some
               C programs and English text.  Only those places
               that contain this "rarest" character are exam­

               For example, here is a loop that inserts index
               producing entries before any line containing a
               certain pattern:

                   while (<>) {
                       print ".IX foo\n"       if /\bfoo\b/;
                       print ".IX bar\n"       if /\bbar\b/;
                       print ".IX blurfl\n"    if /\bblurfl\b/;
                       # ...

               In searching for "/\bfoo\b/", only those locations
               in $_ that contain "f" will be looked at, because
               "f" is rarer than "o".  In general, this is a big
               win except in pathological cases.  The only ques­
               tion is whether it saves you more time than it
               took to build the linked list in the first place.

               Note that if you have to look for strings that you
               don't know till runtime, you can build an entire
               loop as a string and "eval" that to avoid recom­
               piling all your patterns all the time.  Together
               with undefining $/ to input entire files as one
               record, this can be very fast, often faster than
               specialized programs like fgrep(1).  The following
               scans a list of files (@files) for a list of words
               (@words), and prints out the names of those files
               that contain a match:

                   eval $search;               # this screams
                   $/ = "\n";          # put back to normal input delimiter
                   foreach $file (sort keys(%seen)) {
                       print $file, "\n";

       sub NAME BLOCK
       sub NAME (PROTO) BLOCK
       sub NAME : ATTRS BLOCK
               This is subroutine definition, not a real function
               per se.  Without a BLOCK it's just a forward dec­
               laration.  Without a NAME, it's an anonymous func­
               tion declaration, and does actually return a
               value: the CODE ref of the closure you just cre­

               See perlsub and perlref for details about subrou­
               tines and references, and attributes and
               Attribute::Handlers for more information about

       substr EXPR,OFFSET,LENGTH
       substr EXPR,OFFSET
               Extracts a substring out of EXPR and returns it.
               First character is at offset 0, or whatever you've
               set $[ to (but don't do that).  If OFFSET is nega­
               tive (or more precisely, less than $[), starts
               that far from the end of the string.  If LENGTH is
               omitted, returns everything to the end of the
               string.  If LENGTH is negative, leaves that many
               characters off the end of the string.

               You can use the substr() function as an lvalue, in
               which case EXPR must itself be an lvalue.  If you
               assign something shorter than LENGTH, the string
               will shrink, and if you assign something longer
               than LENGTH, the string will grow to accommodate
               it.  To keep the string the same length you may
               need to pad or chop your value using "sprintf".

               If OFFSET and LENGTH specify a substring that is
               partly outside the string, only the part within
               the string is returned.  If the substring is
               beyond either end of the string, substr() returns
               the undefined value and produces a warning.  When
               used as an lvalue, specifying a substring that is
               entirely outside the string is a fatal error.
               Here's an example showing the behavior for bound­
               ary cases:

               be as expected and is subject to change.  This
               caveat includes code such as "print(sub­
               str($foo,$a,$b)=$bar)" or "(sub­
               str($foo,$a,$b)=$bar)=$fud" (where $foo is changed
               via the substring assignment, and then the substr
               is used again), or where a substr() is aliased via
               a "foreach" loop or passed as a parameter or a
               reference to it is taken and then the alias,
               parameter, or deref'd reference either is used
               after the original EXPR has been changed or is
               assigned to and then used a second time.

       symlink OLDFILE,NEWFILE
               Creates a new filename symbolically linked to the
               old filename.  Returns 1 for success, 0 otherwise.
               On systems that don't support symbolic links, pro­
               duces a fatal error at run time.  To check for
               that, use eval:

                   $symlink_exists = eval { symlink("",""); 1 };

       syscall NUMBER, LIST
               Calls the system call specified as the first ele­
               ment of the list, passing the remaining elements
               as arguments to the system call.  If unimple­
               mented, produces a fatal error.  The arguments are
               interpreted as follows: if a given argument is
               numeric, the argument is passed as an int.  If
               not, the pointer to the string value is passed.
               You are responsible to make sure a string is pre-
               extended long enough to receive any result that
               might be written into a string.  You can't use a
               string literal (or other read-only string) as an
               argument to "syscall" because Perl has to assume
               that any string pointer might be written through.
               If your integer arguments are not literals and
               have never been interpreted in a numeric context,
               you may need to add 0 to them to force them to
               look like numbers.  This emulates the "syswrite"
               function (or vice versa):

                   require 'syscall.ph';               # may need to run h2ph
                   $s = "hi there\n";
                   syscall(&SYS_write, fileno(STDOUT), $s, length $s);

               Note that Perl supports passing of up to only 14
               arguments to your system call, which in practice
               should usually suffice.

               Syscall returns whatever value returned by the
               system call it calls.  If the system call fails,
               "syscall" returns "-1" and sets $! (errno).  Note
               NAME, and associates it with FILEHANDLE.  If FILE­
               HANDLE is an expression, its value is used as the
               name of the real filehandle wanted.  This function
               calls the underlying operating system's "open"
               function with the parameters FILENAME, MODE,

               The possible values and flag bits of the MODE
               parameter are system-dependent; they are available
               via the standard module "Fcntl".  See the documen­
               tation of your operating system's "open" to see
               which values and flag bits are available.  You may
               combine several flags using the "|"-operator.

               Some of the most common values are "O_RDONLY" for
               opening the file in read-only mode, "O_WRONLY" for
               opening the file in write-only mode, and "O_RDWR"
               for opening the file in read-write mode, and.

               For historical reasons, some values work on almost
               every system supported by perl: zero means
               read-only, one means write-only, and two means
               read/write.  We know that these values do not work
               under OS/390 & VM/ESA Unix and on the Macintosh;
               you probably don't want to use them in new code.

               If the file named by FILENAME does not exist and
               the "open" call creates it (typically because MODE
               includes the "O_CREAT" flag), then the value of
               PERMS specifies the permissions of the newly cre­
               ated file.  If you omit the PERMS argument to
               "sysopen", Perl uses the octal value 0666.  These
               permission values need to be in octal, and are
               modified by your process's current "umask".

               In many systems the "O_EXCL" flag is available for
               opening files in exclusive mode.  This is not
               locking: exclusiveness means here that if the file
               already exists, sysopen() fails.  The "O_EXCL"
               wins "O_TRUNC".

               Sometimes you may want to truncate an already-
               existing file: "O_TRUNC".

               You should seldom if ever use 0644 as argument to
               "sysopen", because that takes away the user's
               option to have a more permissive umask.  Better to
               omit it.  See the perlfunc(1) entry on "umask" for
               more on this.

               Note that "sysopen" depends on the fdopen() C
               library function.  On many UNIX systems, fdopen()
               so mixing this with other kinds of reads, "print",
               "write", "seek", "tell", or "eof" can cause confu­
               sion because the perlio or stdio layers usually
               buffers data.  Returns the number of bytes actu­
               ally read, 0 at end of file, or undef if there was
               an error (in the latter case $! is also set).
               SCALAR will be grown or shrunk so that the last
               byte actually read is the last byte of the scalar
               after the read.

               An OFFSET may be specified to place the read data
               at some place in the string other than the begin­
               ning.  A negative OFFSET specifies placement at
               that many characters counting backwards from the
               end of the string.  A positive OFFSET greater than
               the length of SCALAR results in the string being
               padded to the required size with "\0" bytes before
               the result of the read is appended.

               There is no syseof() function, which is ok, since
               eof() doesn't work very well on device files (like
               ttys) anyway.  Use sysread() and check for a
               return value for 0 to decide whether you're done.

               Note that if the filehandle has been marked as
               ":utf8" Unicode characters are read instead of
               bytes (the LENGTH, OFFSET, and the return value of
               sysread() are in Unicode characters).  The
               ":encoding(...)" layer implicitly introduces the
               ":utf8" layer.  See "binmode", "open", and the
               "open" pragma, open.

               Sets FILEHANDLE's system position in bytes using
               the system call lseek(2).  FILEHANDLE may be an
               expression whose value gives the name of the file­
               handle.  The values for WHENCE are 0 to set the
               new position to POSITION, 1 to set the it to the
               current position plus POSITION, and 2 to set it to
               EOF plus POSITION (typically negative).

               Note the in bytes: even if the filehandle has been
               set to operate on characters (for example by using
               the ":utf8" I/O layer), tell() will return byte
               offsets, not character offsets (because implement­
               ing that would render sysseek() very slow).

               sysseek() bypasses normal buffered IO, so mixing
               this with reads (other than "sysread", for example
               &gt;&lt or read()) "print", "write", "seek",
               "tell", or "eof" may cause confusion.

               easily determine the new position.

       system LIST
       system PROGRAM LIST
               Does exactly the same thing as "exec LIST", except
               that a fork is done first, and the parent process
               waits for the child process to complete.  Note
               that argument processing varies depending on the
               number of arguments.  If there is more than one
               argument in LIST, or if LIST is an array with more
               than one value, starts the program given by the
               first element of the list with arguments given by
               the rest of the list.  If there is only one scalar
               argument, the argument is checked for shell
               metacharacters, and if there are any, the entire
               argument is passed to the system's command shell
               for parsing (this is "/bin/sh -c" on Unix plat­
               forms, but varies on other platforms).  If there
               are no shell metacharacters in the argument, it is
               split into words and passed directly to "execvp",
               which is more efficient.

               Beginning with v5.6.0, Perl will attempt to flush
               all files opened for output before any operation
               that may do a fork, but this may not be supported
               on some platforms (see perlport).  To be safe, you
               may need to set $| ($AUTOFLUSH in English) or call
               the "autoflush()" method of "IO::Handle" on any
               open handles.

               The return value is the exit status of the program
               as returned by the "wait" call.  To get the actual
               exit value shift right by eight (see below).  See
               also "exec".  This is not what you want to use to
               capture the output from a command, for that you
               should use merely backticks or "qx//", as
               described in "`STRING`" in perlop.  Return value
               of -1 indicates a failure to start the program
               (inspect $! for the reason).

               Like "exec", "system" allows you to lie to a pro­
               gram about its name if you use the "system PROGRAM
               LIST" syntax.  Again, see "exec".

               Because "system" and backticks block "SIGINT" and
               "SIGQUIT", killing the program they're running
               doesn't actually interrupt your program.

                   @args = ("command", "arg1", "arg2");
                   system(@args) == 0
                        or die "system @args failed: $?"

                   else {
                       printf "child exited with value %d\n", $? >> 8;

               or more portably by using the W*() calls of the
               POSIX extension; see perlport for more informa­

               When the arguments get executed via the system
               shell, results and return codes will be subject to
               its quirks and capabilities.  See "`STRING`" in
               perlop and "exec" for details.

       syswrite FILEHANDLE,SCALAR
               Attempts to write LENGTH bytes of data from vari­
               able SCALAR to the specified FILEHANDLE, using the
               system call write(2).  If LENGTH is not specified,
               writes whole SCALAR.  It bypasses buffered IO, so
               mixing this with reads (other than sysread()),
               "print", "write", "seek", "tell", or "eof" may
               cause confusion because the perlio and stdio lay­
               ers usually buffers data.  Returns the number of
               bytes actually written, or "undef" if there was an
               error (in this case the errno variable $! is also
               set).  If the LENGTH is greater than the available
               data in the SCALAR after the OFFSET, only as much
               data as is available will be written.

               An OFFSET may be specified to write the data from
               some part of the string other than the beginning.
               A negative OFFSET specifies writing that many
               characters counting backwards from the end of the
               string.  In the case the SCALAR is empty you can
               use OFFSET but only zero offset.

               Note that if the filehandle has been marked as
               ":utf8", Unicode characters are written instead of
               bytes (the LENGTH, OFFSET, and the return value of
               syswrite() are in UTF-8 encoded Unicode charac­
               ters).  The ":encoding(...)" layer implicitly
               introduces the ":utf8" layer.  See "binmode",
               "open", and the "open" pragma, open.

       tell FILEHANDLE
       tell    Returns the current position in bytes for FILEHAN­
               DLE, or -1 on error.  FILEHANDLE may be an expres­
               sion whose value gives the name of the actual
               filehandle.  If FILEHANDLE is omitted, assumes the
               file last read.

               Do not use tell() on a filehandle that has been
               opened using sysopen(), use sysseek() for that as
               described above.  Why?  Because sysopen() creates
               unbuffered, "raw", filehandles, while open() cre­
               ates buffered filehandles.  sysseek() make sense
               only on the first kind, tell() only makes sense on
               the second kind.

       telldir DIRHANDLE
               Returns the current position of the "readdir" rou­
               tines on DIRHANDLE.  Value may be given to
               "seekdir" to access a particular location in a
               directory.  Has the same caveats about possible
               directory compaction as the corresponding system
               library routine.

               This function binds a variable to a package class
               that will provide the implementation for the vari­
               able.  VARIABLE is the name of the variable to be
               enchanted.  CLASSNAME is the name of a class
               implementing objects of correct type.  Any addi­
               tional arguments are passed to the "new" method of
               the class (meaning "TIESCALAR", "TIEHANDLE",
               "TIEARRAY", or "TIEHASH").  Typically these are
               arguments such as might be passed to the
               "dbm_open()" function of C.  The object returned
               by the "new" method is also returned by the "tie"
               function, which would be useful if you want to
               access other methods in CLASSNAME.

               Note that functions such as "keys" and "values"
               may return huge lists when used on large objects,
               like DBM files.  You may prefer to use the "each"
               function to iterate over such.  Example:

                   # print out history file offsets
                   use NDBM_File;
                   tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
                   while (($key,$val) = each %HIST) {
                       print $key, ' = ', unpack('L',$val), "\n";

               A class implementing a hash should have the fol­
               lowing methods:

                   TIEHASH classname, LIST
                   FETCH this, key
                   STORE this, key, value
                   DELETE this, key
                   STORESIZE this, count
                   CLEAR this
                   PUSH this, LIST
                   POP this
                   SHIFT this
                   UNSHIFT this, LIST
                   SPLICE this, offset, length, LIST
                   EXTEND this, count
                   DESTROY this
                   UNTIE this

               A class implementing a file handle should have the
               following methods:

                   TIEHANDLE classname, LIST
                   READ this, scalar, length, offset
                   READLINE this
                   GETC this
                   WRITE this, scalar, length, offset
                   PRINT this, LIST
                   PRINTF this, format, LIST
                   BINMODE this
                   EOF this
                   FILENO this
                   SEEK this, position, whence
                   TELL this
                   OPEN this, mode, LIST
                   CLOSE this
                   DESTROY this
                   UNTIE this

               A class implementing a scalar should have the fol­
               lowing methods:

                   TIESCALAR classname, LIST
                   FETCH this,
                   STORE this, value
                   DESTROY this
                   UNTIE this

               Not all methods indicated above need be imple­
               mented.  See perltie, Tie::Hash, Tie::Array,
               Tie::Scalar, and Tie::Handle.

               Unlike "dbmopen", the "tie" function will not use
               or require a module for you--you need to do that
               explicitly yourself.  See DB_File or the Config
               module for interesting "tie" implementations.

               For further details see perltie, "tied VARIABLE".

       tied VARIABLE
               second, you may use either the Time::HiRes module
               (from CPAN, and starting from Perl 5.8 part of the
               standard distribution), or if you have gettimeof­
               day(2), you may be able to use the "syscall"
               interface of Perl.  See perlfaq8 for details.

       times   Returns a four-element list giving the user and
               system times, in seconds, for this process and the
               children of this process.

                   ($user,$system,$cuser,$csystem) = times;

               In scalar context, "times" returns $user.

       tr///   The transliteration operator.  Same as "y///".
               See perlop.

       truncate FILEHANDLE,LENGTH
       truncate EXPR,LENGTH
               Truncates the file opened on FILEHANDLE, or named
               by EXPR, to the specified length.  Produces a
               fatal error if truncate isn't implemented on your
               system.  Returns true if successful, the undefined
               value otherwise.

               The behavior is undefined if LENGTH is greater
               than the length of the file.

       uc EXPR
       uc      Returns an uppercased version of EXPR.  This is
               the internal function implementing the "\U" escape
               in double-quoted strings.  Respects current
               LC_CTYPE locale if "use locale" in force.  See
               perllocale and perlunicode for more details about
               locale and Unicode support.  It does not attempt
               to do titlecase mapping on initial letters.  See
               "ucfirst" for that.

               If EXPR is omitted, uses $_.

       ucfirst EXPR
       ucfirst Returns the value of EXPR with the first character
               in uppercase (titlecase in Unicode).  This is the
               internal function implementing the "\u" escape in
               double-quoted strings.  Respects current LC_CTYPE
               locale if "use locale" in force.  See perllocale
               and perlunicode for more details about locale and
               Unicode support.

               If EXPR is omitted, uses $_.

       umask EXPR
               If your "umask" were 0027 (group can't write; oth­
               ers can't read, write, or execute), then passing
               "sysopen" 0666 would create a file with mode 0640
               ("0666 &~ 027" is 0640).

               Here's some advice: supply a creation mode of 0666
               for regular files (in "sysopen") and one of 0777
               for directories (in "mkdir") and executable files.
               This gives users the freedom of choice: if they
               want protected files, they might choose process
               umasks of 022, 027, or even the particularly anti­
               social mask of 077.  Programs should rarely if
               ever make policy decisions better left to the
               user.  The exception to this is when writing files
               that should be kept private: mail files, web
               browser cookies, .rhosts files, and so on.

               If umask(2) is not implemented on your system and
               you are trying to restrict access for yourself
               (i.e., (EXPR & 0700) > 0), produces a fatal error
               at run time.  If umask(2) is not implemented and
               you are not trying to restrict access for your­
               self, returns "undef".

               Remember that a umask is a number, usually given
               in octal; it is not a string of octal digits.  See
               also "oct", if all you have is a string.

       undef EXPR
       undef   Undefines the value of EXPR, which must be an
               lvalue.  Use only on a scalar value, an array
               (using "@"), a hash (using "%"), a subroutine
               (using "&"), or a typeglob (using "*").  (Saying
               "undef $hash{$key}" will probably not do what you
               expect on most predefined variables or DBM list
               values, so don't do that; see delete.)  Always
               returns the undefined value.  You can omit the
               EXPR, in which case nothing is undefined, but you
               still get an undefined value that you could, for
               instance, return from a subroutine, assign to a
               variable or pass as a parameter.  Examples:

                   undef $foo;
                   undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
                   undef @ary;
                   undef %hash;
                   undef &mysub;
                   undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
                   return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
                   select undef, undef, undef, 0.25;
                   ($a, $b, undef, $c) = &foo;       # Ignore third value returned

               that unlinking a directory can inflict damage on
               your filesystem.  Use "rmdir" instead.

               If LIST is omitted, uses $_.

       unpack TEMPLATE,EXPR
               "unpack" does the reverse of "pack": it takes a
               string and expands it out into a list of values.
               (In scalar context, it returns merely the first
               value produced.)

               The string is broken into chunks described by the
               TEMPLATE.  Each chunk is converted separately to a
               value.  Typically, either the string is a result
               of "pack", or the bytes of the string represent a
               C structure of some kind.

               The TEMPLATE has the same format as in the "pack"
               function.  Here's a subroutine that does sub­

                   sub substr {
                       my($what,$where,$howmuch) = @_;
                       unpack("x$where a$howmuch", $what);

               and then there's

                   sub ordinal { unpack("c",$_[0]); } # same as ord()

               In addition to fields allowed in pack(), you may
               prefix a field with a %<number> to indicate that
               you want a <number>-bit checksum of the items
               instead of the items themselves.  Default is a
               16-bit checksum.  Checksum is calculated by sum­
               ming numeric values of expanded values (for string
               fields the sum of "ord($char)" is taken, for bit
               fields the sum of zeroes and ones).

               For example, the following computes the same num­
               ber as the System V sum program:

                   $checksum = do {
                       local $/;  # slurp!
                       unpack("%32C*",<>) % 65535;

               The following efficiently counts the number of set
               bits in a bit vector:

                   $setbits = unpack("%32b*", $selectmask);

               described by the TEMPLATE, the rest is ignored.

               See "pack" for more examples and notes.

       untie VARIABLE
               Breaks the binding between a variable and a pack­
               age.  (See "tie".)  Has no effect if the variable
               is not tied.

       unshift ARRAY,LIST
               Does the opposite of a "shift".  Or the opposite
               of a "push", depending on how you look at it.
               Prepends list to the front of the array, and
               returns the new number of elements in the array.

                   unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;

               Note the LIST is prepended whole, not one element
               at a time, so the prepended elements stay in the
               same order.  Use "reverse" to do the reverse.

       use Module VERSION LIST
       use Module VERSION
       use Module LIST
       use Module
       use VERSION
               Imports some semantics into the current package
               from the named module, generally by aliasing cer­
               tain subroutine or variable names into your pack­
               age.  It is exactly equivalent to

                   BEGIN { require Module; import Module LIST; }

               except that Module must be a bareword.

               VERSION may be either a numeric argument such as
               5.006, which will be compared to $], or a literal
               of the form v5.6.1, which will be compared to $^V
               (aka $PERL_VERSION.  A fatal error is produced if
               VERSION is greater than the version of the current
               Perl interpreter; Perl will not attempt to parse
               the rest of the file.  Compare with "require",
               which can do a similar check at run time.

               Specifying VERSION as a literal of the form v5.6.1
               should generally be avoided, because it leads to
               misleading error messages under earlier versions
               of Perl which do not support this syntax.  The
               equivalent numeric version should be used instead.

                   use v5.6.1;         # compile time version check
                   use 5.6.1;          # ditto

               features back into the current package.  The mod­
               ule can implement its "import" method any way it
               likes, though most modules just choose to derive
               their "import" method via inheritance from the
               "Exporter" class that is defined in the "Exporter"
               module.  See Exporter.  If no "import" method can
               be found then the call is skipped.

               If you do not want to call the package's "import"
               method (for instance, to stop your namespace from
               being altered), explicitly supply the empty list:

                   use Module ();

               That is exactly equivalent to

                   BEGIN { require Module }

               If the VERSION argument is present between Module
               and LIST, then the "use" will call the VERSION
               method in class Module with the given version as
               an argument.  The default VERSION method, inher­
               ited from the UNIVERSAL class, croaks if the given
               version is larger than the value of the variable

               Again, there is a distinction between omitting
               LIST ("import" called with no arguments) and an
               explicit empty LIST "()" ("import" not called).
               Note that there is no comma after VERSION!

               Because this is a wide-open interface, pragmas
               (compiler directives) are also implemented this
               way.  Currently implemented pragmas are:

                   use constant;
                   use diagnostics;
                   use integer;
                   use sigtrap  qw(SEGV BUS);
                   use strict   qw(subs vars refs);
                   use subs     qw(afunc blurfl);
                   use warnings qw(all);
                   use sort     qw(stable _quicksort _mergesort);

               Some of these pseudo-modules import semantics into
               the current block scope (like "strict" or "inte­
               ger", unlike ordinary modules, which import sym­
               bols into the current package (which are effective
               through the end of the file).

               There's a corresponding "no" command that unim­
               ports meanings imported by "use", i.e., it calls
               of the list must be the NUMERICAL access and modi­
               fication times, in that order.  Returns the number
               of files successfully changed.  The inode change
               time of each file is set to the current time.  For
               example, this code has the same effect as the Unix
               touch(1) command when the files already exist.

                   $now = time;
                   utime $now, $now, @ARGV;

               Note:  Under NFS, touch(1) uses the time of the
               NFS server, not the time of the local machine.  If
               there is a time synchronization problem, the NFS
               server and local machine will have different

               Since perl 5.7.2, if the first two elements of the
               list are "undef", then the utime(2) function in
               the C library will be called with a null second
               argument. On most systems, this will set the
               file's access and modification times to the cur­
               rent time (i.e. equivalent to the example above.)

                   utime undef, undef, @ARGV;

       values HASH
               Returns a list consisting of all the values of the
               named hash.  (In a scalar context, returns the
               number of values.)

               The values are returned in an apparently random
               order.  The actual random order is subject to
               change in future versions of perl, but it is guar­
               anteed to be the same order as either the "keys"
               or "each" function would produce on the same
               (unmodified) hash.  Since Perl 5.8.1 the ordering
               is different even between different runs of Perl
               for security reasons (see "Algorithmic Complexity
               Attacks" in perlsec).

               As a side effect, calling values() resets the
               HASH's internal iterator, see "each".

               Note that the values are not copied, which means
               modifying them will modify the contents of the

                   for (values %hash)      { s/foo/bar/g }   # modifies %hash values
                   for (@hash{keys %hash}) { s/foo/bar/g }   # same

               See also "keys", "each", and "sort".
               are grouped into chunks of size BITS/8, and each
               group is converted to a number as with
               pack()/unpack() with big-endian formats "n"/"N"
               (and analogously for BITS==64).  See "pack" for

               If bits is 4 or less, the string is broken into
               bytes, then the bits of each byte are broken into
               8/BITS groups.  Bits of a byte are numbered in a
               little-endian-ish way, as in 0x01, 0x02, 0x04,
               0x08, 0x10, 0x20, 0x40, 0x80.  For example, break­
               ing the single input byte "chr(0x36)" into two
               groups gives a list "(0x6, 0x3)"; breaking it into
               4 groups gives "(0x2, 0x1, 0x3, 0x0)".

               "vec" may also be assigned to, in which case
               parentheses are needed to give the expression the
               correct precedence as in

                   vec($image, $max_x * $x + $y, 8) = 3;

               If the selected element is outside the string, the
               value 0 is returned.  If an element off the end of
               the string is written to, Perl will first extend
               the string with sufficiently many zero bytes.   It
               is an error to try to write off the beginning of
               the string (i.e. negative OFFSET).

               The string should not contain any character with
               the value > 255 (which can only happen if you're
               using UTF-8 encoding).  If it does, it will be
               treated as something which is not UTF-8 encoded.
               When the "vec" was assigned to, other parts of
               your program will also no longer consider the
               string to be UTF-8 encoded.  In other words, if
               you do have such characters in your string, vec()
               will operate on the actual byte string, and not
               the conceptual character string.

               Strings created with "vec" can also be manipulated
               with the logical operators "|", "&", "^", and "~".
               These operators will assume a bit vector operation
               is desired when both operands are strings.  See
               "Bitwise String Operators" in perlop.

               The following code will build up an ASCII string
               saying 'PerlPerlPerl'.  The comments show the
               string after each step.  Note that this code works
               in the same way on big-endian or little-endian

                   my $foo = '';
                   vec($foo, 45,  2) = 3;              # 'PerlPerlPer'  . "\x0c"
                   vec($foo, 93,  1) = 1;              # 'PerlPerlPer'  . "\x2c"
                   vec($foo, 94,  1) = 1;              # 'PerlPerlPerl'
                                                       # 'l' is "\x6c"

               To transform a bit vector into a string or list of
               0's and 1's, use these:

                   $bits = unpack("b*", $vector);
                   @bits = split(//, unpack("b*", $vector));

               If you know the exact length in bits, it can be
               used in place of the "*".

               Here is an example to illustrate how the bits
               actually fall in place:

                   #!/usr/bin/perl -wl

                   print <<'EOT';
                                                     0         1         2         3
                                      unpack("V",$_) 01234567890123456789012345678901

                   for $w (0..3) {
                       $width = 2**$w;
                       for ($shift=0; $shift < $width; ++$shift) {
                           for ($off=0; $off < 32/$width; ++$off) {
                               $str = pack("B*", "0"x32);
                               $bits = (1<<$shift);
                               vec($str, $off, $width) = $bits;
                               $res = unpack("b*",$str);
                               $val = unpack("V", $str);

                   format STDOUT =
                   vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
                   $off, $width, $bits, $val, $res

               Regardless of the machine architecture on which it
               is run, the above example should print the follow­
               ing table:

                   vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
                   vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
                   vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
                   vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
                   vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
                   vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
                   vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
                   vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
                   vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
                   vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
                   vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
                   vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
                   vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
                   vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
                   vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
                   vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
                   vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
                   vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
                   vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
                   vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
                   vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
                   vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
                   vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
                   vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
                   vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
                   vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
                   vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
                   vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
                   vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
                   vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
                   vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
                   vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
                   vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
                   vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
                   vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
                   vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
                   vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
                   vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
                   vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
                   vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
                   vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
                   vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
                   vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
                   vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
                   vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
                   vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
                   vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
                   vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
                   vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
                   vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
                   vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
                   vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
                   vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
                   vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
                   vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
                   vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
                   vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
                   vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
                   vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
                   vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
                   vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
                   vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
                   vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
                   vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
                   vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
                   vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
                   vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
                   vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
                   vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
                   vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
                   vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
                   vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
                   vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
                   vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
                   vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
                   vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
                   vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
                   vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
                   vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
                   vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
                   vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
                   vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
                   vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
                   vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
                   vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
                   vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
                   vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
                   vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
                   vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
                   vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
                   vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
                   vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
                   vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
                   vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
                   vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
                   vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
                   vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
                   vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
                   vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
                   vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
                   vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
                   vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
                   vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
                   vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000

       waitpid PID,FLAGS
               Waits for a particular child process to terminate
               and returns the pid of the deceased process, or
               "-1" if there is no such child process.  On some
               systems, a value of 0 indicates that there are
               processes still running.  The status is returned
               in $?.  If you say

                   use POSIX ":sys_wait_h";
                   do {
                       $kid = waitpid(-1, WNOHANG);
                   } until $kid > 0;

               then you can do a non-blocking wait for all pend­
               ing zombie processes.  Non-blocking wait is avail­
               able on machines supporting either the waitpid(2)
               or wait4(2) system calls.  However, waiting for a
               particular pid with FLAGS of 0 is implemented
               everywhere.  (Perl emulates the system call by
               remembering the status values of processes that
               have exited but have not been harvested by the
               Perl script yet.)

               Note that on some systems, a return value of "-1"
               could mean that child processes are being automat­
               ically reaped.  See perlipc for details, and for
               other examples.

               Returns true if the context of the currently exe­
               cuting subroutine is looking for a list value.
               Returns false if the context is looking for a
               scalar.  Returns the undefined value if the con­
               text is looking for no value (void context).

                   return unless defined wantarray;    # don't bother doing more
                   my @a = complex_calculation();
                   return wantarray ? @a : "@a";

               This function should have been named wantlist()

       warn LIST
               Produces a message on STDERR just like "die", but
               doesn't exit or throw an exception.

               If LIST is empty and $@ already contains a value
               (typically from a previous eval) that value is
               used after appending "\t...caught" to $@.  This is
               useful for staying almost, but not entirely simi­
               since "__WARN__" hooks are not called from inside

               You will find this behavior is slightly different
               from that of $SIG{__DIE__} handlers (which don't
               suppress the error text, but can instead call
               "die" again to change it).

               Using a "__WARN__" handler provides a powerful way
               to silence all warnings (even the so-called manda­
               tory ones).  An example:

                   # wipe out *all* compile-time warnings
                   BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
                   my $foo = 10;
                   my $foo = 20;          # no warning about duplicate my $foo,
                                          # but hey, you asked for it!
                   # no compile-time or run-time warnings before here
                   $DOWARN = 1;

                   # run-time warnings enabled after here
                   warn "\$foo is alive and $foo!";     # does show up

               See perlvar for details on setting %SIG entries,
               and for more examples.  See the Carp module for
               other kinds of warnings using its carp() and
               cluck() functions.

       write FILEHANDLE
       write EXPR
       write   Writes a formatted record (possibly multi-line) to
               the specified FILEHANDLE, using the format associ­
               ated with that file.  By default the format for a
               file is the one having the same name as the file­
               handle, but the format for the current output
               channel (see the "select" function) may be set
               explicitly by assigning the name of the format to
               the $~ variable.

               Top of form processing is handled automatically:
               if there is insufficient room on the current page
               for the formatted record, the page is advanced by
               writing a form feed, a special top-of-page format
               is used to format the new page header, and then
               the record is written.  By default the top-of-page
               format is the name of the filehandle with "_TOP"
               appended, but it may be dynamically set to the
               format of your choice by assigning the name to the
               $^ variable while the filehandle is selected.  The
               number of lines remaining on the current page is
               in variable "$-", which can be set to 0 to force a
               new page.

perl v5.8.1                 2003-09-02                PERLFUNC(1)



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