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

sub intersection{
 my(%count,@res);
...
 @res;
}
@a=intersection(\@b,\@c,\@d);