Beware *a and *b, you're treading near $a and $b, which are sacred to sort.

I think that's a little misleading. $a and $b are used by sort, but otherwise are not special to sort.

Here's the deal:

$a and $b are pre-declared package variables.
That means you can use them -- without declaring them first -- even under strict.

Of course, as with all global variables, safe programming means localizing.

So if there's any chance that you've been called from within a sort (unlikely), localize $a and $b before touching them. And if there's any chance that you will call sort while working with $a or $b, be sure to localize them before passing control to where the sort might happen.