$_{$_} .= '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// );