The Perl Cookbook has some excellent things to say on this topic, specifically in and about recipes 4.6 and 4.7. Hashes should be treated as an array of its keys when doing comparisons like this.

In addition, see the Set::Scalar module, which is specifically designed to do manipulations and tests with sets. In a pinch, though, some code like this might help you (taken directly from the cookbook):

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

@union = keys %union;
@isect = keys %isect;
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If you need the difference:

@diff = ();
foreach $e (keys %union) {
   push(@diff, $e) unless $isect{$e};
}
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Set::Scalar makes this much more straightforward:

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

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