use warnings;

[download]

is short for 

BEGIN {
   require warnings;
   import warnings;
}

[download]

so you could do 

BEGIN {
   my $debug = 1;
   if ($debug) {
      require warnings;
      import warnings;
   }
}

[download]

The if module attempts to clean that up: 

my $debug;
BEGIN { $debug = 1; }
use if $debug, 'warnings';

[download]

In reply to Re: use of "use X" by ikegami
in thread use of "use X" by bbfan

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