He was pointing out that sort {$a cmp $b} @list is slower than sort @list because you save the overhead of calling the sort function. The difference should be big enough at some point that it is faster to reverse sort @list instead of sort {$b cmp $a} @list.

Testing this is going to be tricky, of course, because sorting effiency depends on the order you see elements in.

Also this is a very, very special case. With more complex sorts there is choice about how to write it either way, and reverse has to be a waste of time.

UPDATE
Bloody language maintainers. Going and rendering my hard-won optimization knowledge obsolete by making everything fast. Bah.