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        @union = @intersection = @difference = ();
        %count = ();
...
            push @union, $element;
            push @{ $count{$element} > 1 ? \@intersection : \@differen
+ce }, $element;
        }

        @union = @intersection = @left = @right = ();
        %count = ();
...
        for (keys %common) {
            print "$_\n";
        };