use warnings;
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is short for

BEGIN {
   require warnings;
   import warnings;
}
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so you could do

BEGIN {
   my $debug = 1;
   if ($debug) {
      require warnings;
      import warnings;
   }
}
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The if module attempts to clean that up:

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