Except foo($#a) has me scratching my head. It's marked modifiable, but it's not a reference (at least not obvious to me). Maybe this is the older, darker magic of the aliasing involved with @_ in subs? (Personally, I avoid coding in such an implicit way, preferring to catch the passed arguments explicitly.)
I went back an read most of the thread, and, while much of the discussion was over my head, it seems that those in the know are trying to fix it appropriately (or have already done so).
Thanks to all for a mind-expanding discussion.
Quantum Mechanics: The dreams stuff is made of