Certainly not if $a and $b are lexical, which in this case they are. I'm not even seeing any problem if they are global. Did perl have a problem at some point where the previous values didn't get restored after the sort?

Update: Sorry, I misunderstood that you were warning against using a sort BLOCK in the lexical scope of $a or $b. If you try, perl will die with an error "Can't use "my $[ab]" in sort comparison".