It's what's called the Schwartzian Transform, I believe. You can check it out here: http://www.perlmonks.com/index.pl?node_id=115828 or http://www.perlmonks.com/index.pl?node_id=9108 ...

It's designed to make the sort more efficient, saving on memory and what not ... but don't let me explain it, do a search on the site (there are hundreds of discussions about it)