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

<code perl>
foreach $e (@a) { $union{$e} = 1 }
foreach $e (@b) {
	if ( $union{$e} ) { $isect{$e} = 1 }
	$union{$e} = 1;
}
@union = keys %union;
@isect = keys %isect;
</code>

This would be more idiomatically written as:

<code perl>
foreach $e (@a, @b) { $union{$e}++ && $isect{$e}++ }
@union = keys %union;
@isect = keys %isect;
</code>