The Perl Cookbook has a section on working with pairs of arrays to find intersections, differences, etc.

The following is one piece of code it contains - using a temporary hash:

@a = (1, 3, 5, 6, 7, 8);
@b = (2, 3, 5, 7, 9);

@union = @isect = ();
%union = %isect = ();
%count = ();

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

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