Let me suggest the advice to newbies be stronger than "it's a good idea not to".

Yes, $a and $b can be declared lexically and they work fine. But this trashes their subsequent use in a sort within the same scope.

I wasted half a day recently trying to debug a failing sort. After much gnashing of teeth I finally discovered that $a and $b had been declared and used at an earlier stage in the routine. I changed $a and $b to meaningful names and the later sort ran fine.

My advice: Just say no to "my ($a, $b);" It will work fine in the immediate context but some time in the future you or someone else may decide to sort something at a later point in the same scope -- and there will be much needless weeping and wailing until you discover the problem.