How do I compute the difference of two arrays? How do I compute the intersection of two arrays?

Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array: 

    @union = @intersection = @difference = ();
    %count = ();
    foreach $element (@array1, @array2) { $count{$element}++ }
    foreach $element (keys %count) {
	push @union, $element;
	push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
    }