IIRC the idea was to apply much the same type of logic as occurs with split. Ie the logic that implicitly sets the third argument of the split to be N+1 where N is the number of scalar slots on the LHS of the assignment. I hazzily recall discussion on whether using an explicit slice would also do the same. I think if you trawl the p5p archives for sort and optimize youll find it. I think the basic idea was that

my ($x,$y,$z)=sort @foo;
my @top=(sort @foo)[1..$n];
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would be special cased somehow.

As for your question about XS, I really have no idea right now. Sorry.