use warnings;

##</code><code>##

BEGIN {
   require warnings;
   import warnings;
}

##</code><code>##

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

##</code><code>##

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