And a variation on that, which handles duplicate values within the arrays as well: 
$_{$_} .= 'a' for @foo;
$_{$_} .= 'b' for @bar;

# $_{$key} =~ /ab/ iff $key was in @foo and @bar
#             /a$/ iff $key was only in @foo
#             /^b/ iff $key was only in @bar
# number of occurrences in
#       $foo_plus_bar = length($_{$key});
#       $foo_only   = ( $_{$key} =~ tr/a// );
#       $bar_only   = ( $_{$key} =~ tr/b// );

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Adapting that to handle up to 26 arrays is trivial -- just set a scalar to 'a' for the first array, and increment it for each successive array; using a regex to determine the distribution for a given array value is left as an exercise...

(I updated the last three comment lines to be more consistent with the first three comment lines.)

In reply to Re^2: RFC: The Uniqueness of hashes. by graff
in thread The Uniqueness of hashes. by injunjoel

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