FM / pod

/usr/share/perl5/pod

Name	Size	Mode	Actions
a2p.pod	6107	0644	edit dl rm
perl.pod	15748	0644	edit dl rm
perl56delta.pod	107184	0644	edit dl rm
perl58delta.pod	114908	0644	edit dl rm
perl561delta.pod	124697	0644	edit dl rm
perl570delta.pod	21655	0644	edit dl rm
perl571delta.pod	30382	0644	edit dl rm
perl572delta.pod	25551	0644	edit dl rm
perl573delta.pod	4644	0644	edit dl rm
perl581delta.pod	38060	0644	edit dl rm
perl582delta.pod	4470	0644	edit dl rm
perl583delta.pod	6335	0644	edit dl rm
perl584delta.pod	7363	0644	edit dl rm
perl585delta.pod	5889	0644	edit dl rm
perl586delta.pod	4651	0644	edit dl rm
perl587delta.pod	8357	0644	edit dl rm
perl588delta.pod	25267	0644	edit dl rm
perl589delta.pod	53905	0644	edit dl rm
perl590delta.pod	34951	0644	edit dl rm
perl591delta.pod	11173	0644	edit dl rm
perl592delta.pod	10954	0644	edit dl rm
perl593delta.pod	17034	0644	edit dl rm
perl594delta.pod	12661	0644	edit dl rm
perl595delta.pod	18934	0644	edit dl rm
perl5004delta.pod	56240	0644	edit dl rm
perl5005delta.pod	34285	0644	edit dl rm
perl5100delta.pod	53922	0644	edit dl rm
perl5101delta.pod	43877	0644	edit dl rm
perlaix.pod	18128	0644	edit dl rm
perlamiga.pod	7031	0644	edit dl rm
perlapi.pod	169428	0644	edit dl rm
perlapio.pod	19332	0644	edit dl rm
perlapollo.pod	829	0644	edit dl rm
perlartistic.pod	6802	0644	edit dl rm
perlbeos.pod	2939	0644	edit dl rm
perlbook.pod	663	0644	edit dl rm
perlboot.pod	28355	0644	edit dl rm
perlbot.pod	11675	0644	edit dl rm
perlbs2000.pod	7929	0644	edit dl rm
perlcall.pod	55173	0644	edit dl rm
perlce.pod	9257	0644	edit dl rm
perlcheat.pod	4153	0644	edit dl rm
perlclib.pod	7684	0644	edit dl rm
perlcn.pod	4915	0644	edit dl rm
perlcommunity.pod	6398	0644	edit dl rm
perlcompile.pod	9516	0644	edit dl rm
perlcygwin.pod	28183	0644	edit dl rm
perldata.pod	36879	0644	edit dl rm
perldbmfilter.pod	4987	0644	edit dl rm
perldebguts.pod	31887	0644	edit dl rm
perldebtut.pod	21304	0644	edit dl rm
perldebug.pod	37535	0644	edit dl rm
perldelta.pod	43877	0644	edit dl rm
perldgux.pod	2821	0644	edit dl rm
perldiag.pod	183151	0644	edit dl rm
perldoc.pod	7310	0644	edit dl rm
perldos.pod	10853	0644	edit dl rm
perldsc.pod	25447	0644	edit dl rm
perlebcdic.pod	66607	0644	edit dl rm
perlembed.pod	38033	0644	edit dl rm
perlepoc.pod	3779	0644	edit dl rm
perlfaq.pod	24841	0644	edit dl rm
perlfaq1.pod	17762	0644	edit dl rm
perlfaq2.pod	21614	0644	edit dl rm
perlfaq3.pod	38534	0644	edit dl rm
perlfaq4.pod	81412	0644	edit dl rm
perlfaq5.pod	49201	0644	edit dl rm
perlfaq6.pod	38538	0644	edit dl rm
perlfaq7.pod	37588	0644	edit dl rm
perlfaq8.pod	46128	0644	edit dl rm
perlfaq9.pod	24191	0644	edit dl rm
perlfilter.pod	21140	0644	edit dl rm
perlfork.pod	11440	0644	edit dl rm
perlform.pod	16894	0644	edit dl rm
perlfreebsd.pod	1950	0644	edit dl rm
perlfunc.pod	291909	0644	edit dl rm
perlglossary.pod	112139	0644	edit dl rm
perlgpl.pod	18806	0644	edit dl rm
perlguts.pod	105182	0644	edit dl rm
perlhack.pod	121512	0644	edit dl rm
perlhaiku.pod	1504	0644	edit dl rm
perlhist.pod	38766	0644	edit dl rm
perlhpux.pod	28650	0644	edit dl rm
perlhurd.pod	1990	0644	edit dl rm
perlintern.pod	26408	0644	edit dl rm
perlintro.pod	21491	0644	edit dl rm
perliol.pod	33755	0644	edit dl rm
perlipc.pod	67761	0644	edit dl rm
perlirix.pod	4397	0644	edit dl rm
perljp.pod	8070	0644	edit dl rm
perlko.pod	7892	0644	edit dl rm
perllexwarn.pod	14435	0644	edit dl rm
perllinux.pod	1492	0644	edit dl rm
perllocale.pod	41616	0644	edit dl rm
perllol.pod	8252	0644	edit dl rm
perlmachten.pod	4500	0644	edit dl rm
perlmacos.pod	2111	0644	edit dl rm
perlmacosx.pod	11322	0644	edit dl rm
perlmint.pod	9539	0644	edit dl rm
perlmod.pod	24494	0644	edit dl rm
perlmodinstall.pod	13929	0644	edit dl rm
perlmodlib.pod	77333	0644	edit dl rm
perlmodstyle.pod	21114	0644	edit dl rm
perlmpeix.pod	15087	0644	edit dl rm
perlmroapi.pod	2959	0644	edit dl rm
perlnetware.pod	6488	0644	edit dl rm
perlnewmod.pod	11214	0644	edit dl rm
perlnumber.pod	8352	0644	edit dl rm
perlobj.pod	21813	0644	edit dl rm
perlop.pod	96225	0644	edit dl rm
perlopenbsd.pod	1207	0644	edit dl rm
perlopentut.pod	37997	0644	edit dl rm
perlos2.pod	92783	0644	edit dl rm
perlos390.pod	16074	0644	edit dl rm
perlos400.pod	4618	0644	edit dl rm
perlothrtut.pod	40657	0644	edit dl rm
perlpacktut.pod	51047	0644	edit dl rm
perlperf.pod	51328	0644	edit dl rm
perlplan9.pod	5125	0644	edit dl rm
perlpod.pod	21626	0644	edit dl rm
perlpodspec.pod	67792	0644	edit dl rm
perlport.pod	86302	0644	edit dl rm
perlpragma.pod	4320	0644	edit dl rm
perlqnx.pod	4246	0644	edit dl rm
perlre.pod	82798	0644	edit dl rm
perlreapi.pod	25427	0644	edit dl rm
perlrebackslash.pod	20109	0644	edit dl rm
perlrecharclass.pod	21909	0644	edit dl rm
perlref.pod	26465	0644	edit dl rm
perlreftut.pod	18670	0644	edit dl rm
perlreguts.pod	36877	0644	edit dl rm
perlrepository.pod	23453	0644	edit dl rm
perlrequick.pod	17617	0644	edit dl rm
perlreref.pod	11996	0644	edit dl rm
perlretut.pod	115005	0644	edit dl rm
perlriscos.pod	1511	0644	edit dl rm
perlrun.pod	49928	0644	edit dl rm
perlsec.pod	23542	0644	edit dl rm
perlsolaris.pod	29131	0644	edit dl rm
perlstyle.pod	8618	0644	edit dl rm
perlsub.pod	54222	0644	edit dl rm
perlsymbian.pod	16222	0644	edit dl rm
perlsyn.pod	31090	0644	edit dl rm
perlthrtut.pod	46510	0644	edit dl rm
perltie.pod	36665	0644	edit dl rm
perltoc.pod	642987	0644	edit dl rm
perltodo.pod	48360	0644	edit dl rm
perltooc.pod	51425	0644	edit dl rm
perltoot.pod	68625	0644	edit dl rm
perltrap.pod	41151	0644	edit dl rm
perltru64.pod	7736	0644	edit dl rm
perltw.pod	5384	0644	edit dl rm
perlunicode.pod	56169	0644	edit dl rm
perlunifaq.pod	12878	0644	edit dl rm
perluniintro.pod	32973	0644	edit dl rm
perlunitut.pod	7907	0644	edit dl rm
perlutil.pod	9881	0644	edit dl rm
perluts.pod	3180	0644	edit dl rm
perlvar.pod	59133	0644	edit dl rm
perlvmesa.pod	3952	0644	edit dl rm
perlvms.pod	52545	0644	edit dl rm
perlvos.pod	5567	0644	edit dl rm
perlwin32.pod	40809	0644	edit dl rm
perlxs.pod	74194	0644	edit dl rm
perlxstut.pod	49670	0644	edit dl rm

Edit: /usr/share/perl5/pod/perllexwarn.pod (14435B)

=head1 NAME
X<warning, lexical> X<warnings> X<warning>

perllexwarn - Perl Lexical Warnings

=head1 DESCRIPTION

The C<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 B<-w> and the equivalent Perl
variable, C<$^W>.

This pragma works just like the C<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 C<use>, C<require> or C<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<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
warning, but the assignment to the scalar C<$b> will not.

=head2 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 C<"isn't numeric">
warning about the "2:".

my $a = "2:" + 3;

With the introduction of lexical warnings, mandatory warnings now become
I<default> warnings. The difference is that although the previously
mandatory warnings are still enabled by default, they can then be
subsequently enabled or disabled with the lexical warning pragma. For
example, in the code below, an C<"isn't numeric"> warning will only
be reported for the C<$a> variable.

my $a = "2:" + 3;
    no warnings;
    my $b = "2:" + 3;

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

=head2 What's wrong with B<-w> and C<$^W>

Although very useful, the big problem with using B<-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 B<-w> flag in this case, you
end up enabling warnings in pieces of code that you haven't written.

Similarly, using C<$^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 B<-w> flag, a warning will be produced
for the C<$a> line -- C<"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 C<$^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 B<-w> flag), the second call
to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
the first will not.

sub doit
    {
        my $b; chop $b;
    }

doit();

{
        local ($^W) = 1;
        doit()
    }

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

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

=head2 Controlling Warnings from the Command Line

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

=over 5

=item B<-w>
X<-w>

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

=item B<-W>
X<-W>

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

=item B<-X>
X<-X>

Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.

=back

=head2 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 C<$^W>, this section will describe how they interact.

How Lexical Warnings interact with B<-w>/C<$^W>:

=over 5

=item 1.

If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
control warnings is used and neither C<$^W> or the C<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.

=item 2.

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

=item 3.

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

=item 4.

If a piece of code is under the control of the C<warnings> pragma,
both the C<$^W> variable and the B<-w> flag will be ignored for the
scope of the lexical warning.

=item 5.

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

=back

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

=head2 Category Hierarchy
X<warning, categories>

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

The current hierarchy is:

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
C<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
L<perldiag>.

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.

=head2 Fatal Warnings
X<warning, fatal>

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 C<time>, C<length>
and C<join> can all produce a C<"Useless use of xxx in void context">
warning.

use warnings;

time;

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

join "", 1,2,3;

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 C<length> is used has escalated the C<void> warnings
category into a fatal error, so the program terminates 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';

=head2 Reporting Warnings from a Module
X<warning, reporting> X<warning, registering>

The C<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 C<warnings>
pragma.

Consider the module C<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 C<warnings::register> will create a new warnings category
called "MyMod::abc", i.e. the new category name matches the current
package name. The C<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 C<MyMod::Abc> has actually
enabled them with the C<warnings> pragma like below.

use MyMod::Abc;
    use warnings 'MyMod::Abc';
    ...
    abc::open("../fred.txt");

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

package MyMod::Abc;

sub open {
        warnings::warnif("deprecated", 
                         "open is deprecated, use new instead");
        new(@_);
    }

sub new
    ...
    1;

The function C<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 C<warnings::warn> or C<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';
    ...
    MyMod::Abc::open('../fred.txt');

the C<warnings::warnif> function will detect this and die after
displaying the warning message.

The three warnings functions, C<warnings::warn>, C<warnings::warnif>
and C<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;
        $self->check($value);
        # ...
    }

package Derived;

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

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

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

When this code is run only the C<Derived> object, C<$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.

=head1 SEE ALSO

L<warnings>, L<perldiag>.

=head1 AUTHOR

Paul Marquess

cmd: