Use Hashes for Sets

• Consider finding the union and intersection of two unique arrays @a and @b:

      foreach $e (@a) { $union{$e} = 1 }
      foreach $e (@b) {
          if ( $union{$e} ) { $isect{$e} = 1 }
          $union{$e} = 1;
      }
      @union = keys %union;
      @isect = keys %isect;