foreach $e (@a, @b) { $union{$e}++ && $isect{$e}++ }

@union = keys %union;
@isect = keys %isect;

##</code><code>##

@diff = ();
foreach $e (keys %union) {
   push(@diff, $e) unless $isect{$e};
}

##</code><code>##

$s = new Set::Scalar (keys %hash1);
$t = new Set::Scalar (keys %hash2);

@isect = $s->intersection($t)->members;