better union of sets algorithm?

perrin has asked for the wisdom of the Perl Monks concerning the following question:

We all know the classic perl solutions for finding the unique elements in a list, as described in perlfaq4's "How can I remove duplicate elements from a list or array?" For your benefit:

    undef %saw;
    @out = grep(!$saw{$_}++, @in);
[download]

I have multiple large sets, which I need to find the union of. I'm wondering if there's anything better than this standard approach, where better means faster. (But little tweaks with "exists" or hash-slices that don't change the basic algorithm aren't enough to be interesting.) It seems unlikely, but I thought maybe the fact that I can get the individual sets sorted ahead of time might allow some sneaky trick. Any ideas?

Back to Seekers of Perl Wisdom