Making it optional may not help the implementation difficulties much. Any object requiring prompt finalization may be pointed to by something buried deep within a graph of objects that don't care, so you still have to check for the final reference being lost even when you're only manipulating objects that don't care.

Your locked open file example is a good one, I think, although I wonder how many times it could be handled with a scope exit action (how often will you pass it to some routine that could keep a global handle on it? If you're doing that, then you're probably not too concerned with the locking issue...)

The window example can certainly be handled with a scope exit mechanism -- you wouldn't be coding the cleanup down at the bottom of the block, you'd be inserting it into a CLEANUP{} block or something that the compiler would be required to call no matter how the block is exited.

Your gc sweep at the end of a block idea works, except for the case when the variable escapes the block (eg it's referenced by a global cache.) Then you'd need to remember that there is at least one rogue object that requires timely finalization floating around, and you'd have to trigger a sweep at every scope exit. Or worse, if you really want it immediate -- simple variable assignment drops a reference to the previous contents, so perl5 can trigger DESTROY even then. A sweep after every assignment might be a little slow...