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Wiki MarkupPerl provides several mechanisms for warning the user about potential problems with the program. The {{use warnings}} pragma turns on a default set of warnings for the Perl runtime to produce should it detect questionable code. The {{-w}} command-line argument serves the same purpose. It is considered so useful that the {{perl(1)}} manpage \[[Wall 2011|AA. Bibliography#Manpages]\] dryly notes the first bug in Perl is that :

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"the -w switch is not mandatory" [Wall 2011] .

The use warnings pragma must be used in all Perl code.

One of the modules that Perl provides for additional safety is the strict module. It identifies programming constructs that are likely to be errors, such as unqualified and undeclared variables (that might be typos), dangerous references, and unqualified subroutine names. The use strict pragma must also be used in all Perl code.

However, occasionally there is a need to disable warnings or strictness for some code that may look strange , but is actually correct. The -w switch cannot enable or disable particular warnings on particular ranges of code. When a particular warning or strict checking must be disabled, the no warnings or no strict pragmas should be used in as minimal a scope as possible. They should also disable the specific warning or strictness checker that would trigger a warning or fatal error , rather than disable all checks.

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use warnings;
use strict;

my %days = ("Sunday"    => 'pray',
            "Monday"    => 'work',
            "Tuesday"   => 'work',
            "Wednesday" => 'work',
            "Thursday"  => 'work',
            "Friday"    => 'work',
            "Saturday"  => 'rest');

sub what_to_do {
  my $day = shift;
  if ($days{$day} eq 'work') {
    return 'work hard';
  }
  if (exists $days{$day}) {
    return $days{$day};
  } else {
    return "do nothing";
  }
}


my $task = what_to_do('tomorrow');
print "Prepare to $task\n";

This code produces the following output:

Use of uninitialized value within %days in string eq at ./example.pl line 16.
Prepare to do nothing

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use warnings;
use strict;
no warnings 'uninitialized';

my %days = ("Sunday"    => 'pray',
# ...

Unfortunately while , although this does code correctly suppress suppresses the warning message, it has the undesired effect of suppressing the warning message throughout the entire program . This has the likely effect of suppressing and will likely suppress the warning in other lines of code that are not known to be correct.

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sub what_to_do {
  my $day = shift;
  no warnings 'uninitialized';
  if ($days{$day} eq 'work') {
    return 'work hard';
  }
  if (exists $days{$day}) {
    return $days{$day};
  } else {
    return "do nothing";
  }
}

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use strict;
use warnings;

our $sunday    = 'pray';
our $monday    = 'work';
our $tuesday   = 'work';
our $wednesday = 'work';
our $thursday  = 'work';
our $friday    = 'work';
our $saturday  = 'rest';

sub what_to_do {
  my $day = shift;
  no warnings 'uninitialized';
  if ($$day eq 'work') {
    return 'work hard';
  }
  if (defined $$day) {
    return $$day;
  } else {
    return "do nothing";
  }
}


my $task = what_to_do('tomorrow');
print "Prepare to $task\n";

The strict pragma catches the improper reference and aborts the program, producing the following error message:

Can't use string ("tomorrow") as a SCALAR ref while "strict refs" in use at ./example.pl line 19.

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This noncompliant code example disables the strict pragma, thus producing proper output. However, strict-ness strictness is suppressed throughout the entire program.

use warnings;
use strict;

no strict 'refs';

our $sunday    = 'pray';
# ...

This code produces the following output:

Prepare to do nothing

This example may be considered correct, but the code works by referencing a nonexistent variable $tomorrow.

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This compliant solution suppresses the strict-ness strictness checking to as minimal a scope as possible. Because the strict strictness checking is suppressed only inside the what_to_do subroutine, other regions of the code can still be checked for strict compliance.

sub what_to_do {
  my $day = shift;
  no warnings 'uninitialized';
  no strict 'refs';
  if ($$day eq 'work') {
    return 'work hard';
  }
  if (defined $$day) {
    return $$day;
  } else {
    return "do nothing";
  }
}

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Related Guidelines

CERT C Secure Coding Standard: MSC00-C. Compile cleanly at high warning levels

CERT C++ Secure Coding Standard: MSC00-CPP. Compile cleanly at high warning levels

Automated Detection

Bibliography

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Related Guidelines

Bibliography

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Image Added Image Added |http://search.cpan.org/dist/Perl-Critic/lib/Perl/Critic/Policy/TestingAndDebugging/ProhibitNoWarnngs.pm], [TestingAndDebugging::ProhibitProlongedStrictureOverride|http://search.cpan.org/dist/Perl-Critic/lib/Perl/Critic/Policy/TestingAndDebugging/ProhibitProlongedStrictureOverride.pm], [TestingAndDebugging::RequireUseStrict|http://search.cpan.org/dist/Perl-Critic/lib/Perl/Critic/Policy/TestingAndDebugging/RequireUseStrict.pm] \[[Wall 2011|AA. Bibliography#Manpages]\] [perl|http://perldoc.perl.org/perl.html] [strict|http://perldoc.perl.org/strict.html] [warnings|http://perldoc.perl.org/warnings.html]EXP30-PL. Do not use deprecated or obsolete functions      02. Expressions      Image Modified