my %seen;
  foreach my $item ( @list ) {
    return error() if ( $seen{$item}++ );
  }
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UPDATE No - that tells you if the item is in the same list twice. :(

  my %seen;
  $seen{@list} = (1) x @list;
  foreach my $item ( @list2 ) {
     return error() if ( $seen{$item} );
  }
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