It doesn't increment after returning. It increments first, and then returns the old value.

No, and that's the entire problem. You do not know when the increment will happen, that is specifically left undefined. (It was left undefined in C to give implementers of compilers all sorts of freedom to optimize it - idem for Perl). What a specific version of Perl does not gives you absolutely no garantee what it will do in a next version.

Abigail