On each iteration of the loop the (simplified) process is
- Allocate memory for @array
- Initialize @array
- Deallocate memory assigned to @array
Well, I don't know what you mean by "Initialize @array", but otherwise, that's the documented behaviour. And in most cases — including the one you called optimized — it's the observable behaviour.
Perl, however, is not that simple and its loop would be more like
- Allocate memory for @array
- Initialize @array
- Deallocate memory assigned to @array if not referred to else where
No, that's not what it does. For starters, the @array is never deallocated by the loop (or sub).
- Allocate memory for @array if it isn't already allocated. (I don't actually know when this is done. It might be earlier.)
- When my is executed, put a directive on the stack to clear/free @array.
- On stack unwind, in response to the directive, clear the @array if there are no more references to it. Otherwise, allocate a new @array.