The "use warnings" pragma enables to control precisely what warnings
       are to be enabled in which parts of a Perl program. It's a more
       flexible alternative for both the command line flag -w and the
       equivalent Perl variable, $^W.

       This pragma works just like the "strict" pragma.  This means that the
       scope of the warning pragma is limited to the enclosing block. It also
       means that the pragma setting 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 module.

       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 "isn't numeric" warning about the

           my $a = "2:" + 3;

       With the introduction of lexical warnings, mandatory warnings now
       become default warnings. The difference is that although the previously

   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 nothing. 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 written.

       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

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

       The other big problem with $^W is the way you can inadvertently change
       the warning setting in unexpected places in your code. For example,
       when the code below is run (without 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;


               local ($^W) = 1;

       This is a side-effect of $^W being dynamically scoped.

       Lexical warnings get around these limitations by allowing finer control
       over where warnings can or can't be tripped.

            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 disables all

   Backward Compatibility
       If you are used to 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 nor 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 control 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 currently 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 "warnings" 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.

       The combined effect of 3 & 4 is that it will allow code which uses the
       "warnings" pragma to control the warning behavior of $^W-type code
       (using a "local $^W=0") if it really wants to, but not vice-versa.

   Category Hierarchy
       A hierarchy of "categories" have been defined to allow groups of
       warnings to be enabled/disabled in isolation.

       The current hierarchy is:

           all -+
                +- closure
                +- deprecated
                +- exiting
                +- experimental --+
                |                 |
                |                 +- layer
                |                 |
                |                 +- newline
                |                 |
                |                 +- pipe
                |                 |
                |                 +- unopened
                +- misc
                +- numeric
                +- once
                +- overflow
                +- pack
                +- portable
                +- recursion
                +- redefine
                +- regexp
                +- severe --------+
                |                 |
                |                 +- debugging
                |                 |
                |                 +- inplace
                |                 |
                |                 +- internal
                |                 |
                |                 +- malloc
                +- signal
                +- substr
                +- syntax --------+
                |                 |
                |                 +- ambiguous
                |                 |
                |                 +- bareword
                |                 |
                |                 +- digit
                |                 |
                |                 +- illegalproto
                |                 |
                |                 +- parenthesis
                |                 |
                |                 +- precedence
                |                 |
                +- threads
                +- uninitialized
                +- unpack
                +- untie
                +- utf8 ----------+
                |                 |
                |                 +- non_unicode
                |                 |
                |                 +- nonchar
                |                 |
                |                 +- surrogate
                +- void

       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

           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

       Note: In Perl 5.6.1, the lexical warnings category "deprecated" 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 specified in the lexical scope
       into fatal errors. In the code below, the use of "time", "length" and
       "join" can all produce a "Useless use of xxx in void context" warning.

           use warnings;


               use warnings FATAL => qw(void);
               length "abc";

       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 module 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 /var/abc")
                       if warnings::enabled();
                   $path = "/var/abc/$path";


       The call to "warnings::register" will create a new warnings category
       called "MyMod::Abc", i.e. the new category name matches the current
       package name. The "open" function in the module will display a warning
       message if it gets given a relative path as a parameter. This warnings
       will only be displayed if the code that uses "MyMod::Abc" has actually
       enabled them with the "warnings" pragma like below.

           use MyMod::Abc;
           use warnings 'MyMod::Abc';

       It is also possible to test whether the pre-defined warnings categories
       are set in the calling module with the "warnings::enabled" function.
       Consider this snippet of code:

       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;

       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", "warnings::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 new
               my $class = shift;
               bless [], $class;

           sub check
               my $self = shift;
               my $value = shift;

               if ($value % 2 && warnings::enabled($self))
                 { warnings::warn($self, "Odd numbers are unsafe") }

           sub doit
               my $self = shift;
               my $value = shift;

               my $class = shift;
               bless [], $class;


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

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

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

           Odd numbers are unsafe at line 7

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

       When registering new categories of warning, you can supply more names
       to warnings::register like this:

           package MyModule;
           use warnings::register qw(format precision);


           warnings::warnif('MyModule::format', '...');

       warnings, perldiag.

       Paul Marquess

perl v5.18.2                      2014-01-06                    PERLLEXWARN(1)
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