You forgot to import. 
BEGIN {
   use constant DEBUG => 1;
   if (DEBUG) {
      require Data::Dumper;
      Data::Dumper->import() if Data::Dumper->can('import');
   }
}

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In reply to Re^2: syntax question: conditional 'use' statement? by ikegami
in thread syntax question: conditional 'use' statement? by Anonymous Monk

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