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       The "use warnings" pragma is a replacement for both the
       command line flag -w and the equivalent Perl variable,

       The pragma works just like the existing "strict" pragma.
       This means that the scope of the warning pragma is limited
       to the enclosing block. It also means that the pragma set­
       ting will not leak across files (via "use", "require" or
       "do"). This allows authors to independently define the
       degree of warning checks that will be applied to their

       By default, optional warnings are disabled, so any legacy
       code that doesn't attempt to control the warnings will
       work unchanged.

       All warnings are enabled in a block by either of these:

           use warnings ;
           use warnings 'all' ;

       Similarly all warnings are disabled in a block by either
       of these:

           no warnings ;
           no warnings 'all' ;

       For example, consider the code below:

           use warnings ;
           my @a ;
               no warnings ;
               my $b = @a[0] ;
           my $c = @a[0];

       The code in the enclosing block has warnings enabled, but
       the inner block has them disabled. In this case that means
       the assignment to the scalar $c will trip the "Scalar
       value @a[0] better written as $a[0]" warning, but the
       assignment to the scalar $b will not.

       Default Warnings and Optional Warnings

       Before the introduction of lexical warnings, Perl had two
       classes of warnings: mandatory and optional.

       As its name suggests, if your code tripped a mandatory
       warning, you would get a warning whether you wanted it or
       not.  For example, the code below would always produce an
           my $b = "2:" + 3;

       Note that neither the -w flag or the $^W can be used to
       disable/enable default warnings. They are still mandatory
       in this case.

       What's wrong with -w and $^W

       Although very useful, the big problem with using -w on the
       command line to enable warnings is that it is all or noth­
       ing. Take the typical scenario when you are writing a Perl
       program. Parts of the code you will write yourself, but
       it's very likely that you will make use of pre-written
       Perl modules. If you use the -w flag in this case, you end
       up enabling warnings in pieces of code that you haven't

       Similarly, using $^W to either disable or enable blocks of
       code is fundamentally flawed. For a start, say you want to
       disable warnings in a block of code. You might expect this
       to be enough to do the trick:

                local ($^W) = 0 ;
                my $a =+ 2 ;
                my $b ; chop $b ;

       When this code is run with the -w flag, a warning will be
       produced for the $a line -- "Reversed += operator".

       The problem is that Perl has both compile-time and run-
       time warnings. To disable compile-time warnings you need
       to rewrite the code like this:

                BEGIN { $^W = 0 }
                my $a =+ 2 ;
                my $b ; chop $b ;

       The other big problem with $^W is the way you can inadver­
       tently change the warning setting in unexpected places in
       your code. For example, when the code below is run (with­
       out the -w flag), the second call to "doit" will trip a
       "Use of uninitialized value" warning, whereas the first
       will not.

           sub doit
               my $b ; chop $b ;

       There are three Command Line flags that can be used to
       control when warnings are (or aren't) produced:

       -w   This is  the existing flag. If the lexical warnings
            pragma is not used in any of you code, or any of the
            modules that you use, this flag will enable warnings
            everywhere. See "Backward Compatibility" for details
            of how this flag interacts with lexical warnings.

       -W   If the -W flag is used on the command line, it will
            enable all warnings throughout the program regardless
            of whether warnings were disabled locally using "no
            warnings" or "$^W =0". This includes all files that
            get included via "use", "require" or "do".  Think of
            it as the Perl equivalent of the "lint" command.

       -X   Does the exact opposite to the -W flag, i.e. it dis­
            ables all warnings.

       Backward Compatibility

       If you are used with working with a version of Perl prior
       to the introduction of lexically scoped warnings, or have
       code that uses both lexical warnings and $^W, this section
       will describe how they interact.

       How Lexical Warnings interact with -w/$^W:

       1.   If none of the three command line flags (-w, -W or
            -X) that control warnings is used and neither $^W or
            the "warnings" pragma are used, then default warnings
            will be enabled and optional warnings disabled.  This
            means that legacy code that doesn't attempt to con­
            trol the warnings will work unchanged.

       2.   The -w flag just sets the global $^W variable as in
            5.005 -- this means that any legacy code that cur­
            rently relies on manipulating $^W to control warning
            behavior will still work as is.

       3.   Apart from now being a boolean, the $^W variable
            operates in exactly the same horrible uncontrolled
            global way, except that it cannot disable/enable
            default warnings.

       4.   If a piece of code is under the control of the "warn­
            ings" pragma, both the $^W variable and the -w flag
            will be ignored for the scope of the lexical warning.

       5.   The only way to override a lexical warnings setting
            is with the -W or -X command line flags.

              +- glob
              +- io -----------+
              |                |
              |                +- closed
              |                |
              |                +- exec
              |                |
              |                +- layer
              |                |
              |                +- newline
              |                |
              |                +- pipe
              |                |
              |                +- unopened
              +- misc
              +- numeric
              +- once
              +- overflow
              +- pack
              +- portable
              +- recursion
              +- redefine
              +- regexp
              +- severe -------+
              |                |
              |                +- debugging
              |                |
              |                +- inplace
              |                |
              |                +- internal
              |                |
              |                +- malloc
              +- signal
              +- substr
              +- syntax -------+
              |                |
              |                +- ambiguous
              |                |
              |                +- reserved
              |                |
              |                +- semicolon
              +- taint
              +- threads
              +- uninitialized
              +- unpack
              +- untie
              +- utf8
              +- void
              +- y2k

       Just like the "strict" pragma any of these categories can
       be combined

           use warnings qw(void redefine) ;
           no warnings qw(io syntax untie) ;

       Also like the "strict" pragma, if there is more than one
       instance of the "warnings" pragma in a given scope the
       cumulative effect is additive.

           use warnings qw(void) ; # only "void" warnings enabled
           use warnings qw(io) ;   # only "void" & "io" warnings enabled
           no warnings qw(void) ;  # only "io" warnings enabled

       To determine which category a specific warning has been
       assigned to see perldiag.

       Note: In Perl 5.6.1, the lexical warnings category "depre­
       cated" was a sub-category of the "syntax" category. It is
       now a top-level category in its own right.

       Fatal Warnings

       The presence of the word "FATAL" in the category list will
       escalate any warnings detected from the categories speci­
       fied in the lexical scope into fatal errors. In the code
       below, the use of "time", "length" and "join" can all pro­
       duce a "Useless use of xxx in void context" warning.

           print "done\n" ;

       When run it produces this output

           Useless use of time in void context at fatal line 3.
           Useless use of length in void context at fatal line 7.

       The scope where "length" is used has escalated the "void"
       warnings category into a fatal error, so the program ter­
       minates immediately it encounters the warning.

       To explicitly turn off a "FATAL" warning you just disable
       the warning it is associated with.  So, for example, to
       disable the "void" warning in the example above, either of
       these will do the trick:

           no warnings qw(void);
           no warnings FATAL => qw(void);

       If you want to downgrade a warning that has been escalated
       into a fatal error back to a normal warning, you can use
       the "NONFATAL" keyword. For example, the code below will
       promote all warnings into fatal errors, except for those
       in the "syntax" category.

           use warnings FATAL => 'all', NONFATAL => 'syntax';

       Reporting Warnings from a Module

       The "warnings" pragma provides a number of functions that
       are useful for module authors. These are used when you
       want to report a module-specific warning to a calling mod­
       ule has enabled warnings via the "warnings" pragma.

       Consider the module "MyMod::Abc" below.

           package MyMod::Abc;

           use warnings::register;

           sub open {
               my $path = shift ;
               if ($path !~ m#^/#) {
                   warnings::warn("changing relative path to /tmp/")
                       if warnings::enabled();
                   $path = "/tmp/$path" ;

           1 ;

       The call to "warnings::register" will create a new warn­
       ings categories are set in the calling module with the
       "warnings::enabled" function. Consider this snippet of

           package MyMod::Abc;

           sub open {
                                "open is deprecated, use new instead") ;
               new(@_) ;

           sub new
           1 ;

       The function "open" has been deprecated, so code has been
       included to display a warning message whenever the calling
       module has (at least) the "deprecated" warnings category
       enabled. Something like this, say.

           use warnings 'deprecated';
           use MyMod::Abc;
           MyMod::Abc::open($filename) ;

       Either the "warnings::warn" or "warnings::warnif" function
       should be used to actually display the warnings message.
       This is because they can make use of the feature that
       allows warnings to be escalated into fatal errors. So in
       this case

           use MyMod::Abc;
           use warnings FATAL => 'MyMod::Abc';

       the "warnings::warnif" function will detect this and die
       after displaying the warning message.

       The three warnings functions, "warnings::warn", "warn­
       ings::warnif" and "warnings::enabled" can optionally take
       an object reference in place of a category name. In this
       case the functions will use the class name of the object
       as the warnings category.

       Consider this example:

           package Original ;

           no warnings ;
           use warnings::register ;

           sub doit
               my $self = shift ;
               my $value = shift ;
               $self->check($value) ;
               # ...

           1 ;

           package Derived ;

           use warnings::register ;
           use Original ;
           our @ISA = qw( Original ) ;
           sub new
               my $class = shift ;
               bless [], $class ;

           1 ;

       The code below makes use of both modules, but it only
       enables warnings from "Derived".

           use Original ;
           use Derived ;
           use warnings 'Derived';
           my $a = new Original ;
           $a->doit(1) ;
           my $b = new Derived ;
           $a->doit(1) ;

       When this code is run only the "Derived" object, $b, will
       generate a warning.

           Odd numbers are unsafe at main.pl line 7

       Notice also that the warning is reported at the line where
       the object is first used.


           The debugger saves and restores C<$^W> at runtime. I haven't checked
           whether the debugger will still work with the lexical warnings
           patch applied.

           I *think* I've got diagnostics to work with the lexical warnings

perl v5.8.1                 2003-09-02             PERLLEXWARN(1)
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