While I fully agree with Marshall that prototypes should generally be avoided (that is, unless you really know what your doing and why), and with you that, similarly, $a and $b should also be avoided most of the time, I think that we shouldn't be too dogmatic about that and that it does make sense to use both prototypes and the $a and $b special variables if you're going to write your own sort subroutine.

BTW, the List::Util module documentation illustrates some cases where it makes perfect sense to use the $a and $b variables in a context that has nothing to do with sorting.

In the example that I gave in Re: Prototype like sort()?, I'm localizing $a and $b before using them to avoid (at least, I hope) any side effects with their use somewhere else.