Actually, thinking about it, that's the disjunction of two sets and there's probably some really short recipe to do it in the Cookbook.

ITYM FAQ, but yes.

$ perldoc -q 'difference'
Found in /usr/lib/perl5/5.6.1/pod/perlfaq4.pod
       How do I compute the difference of two arrays?  How do I
       compute the intersection of two arrays?
...
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