Unlike *_, *ENV, etc, $a and $b are in the current package. They are just, like @INC, only exempt from strict.

Beware! Should you wish to write a sort subroutine that will be called from another package, you must go out of your way to use $a and $b from *their* package, and not your own. (Similarly if they try to pass a sort subroutine into your function, it won't work as expected.)

UPDATE
Thanks tye for spotting my obvious thinko.