You're asking if there's a way to get notified if my or local hides an existing variable.

Interesting idea...

However, at least in the case of local, that's its main _purpose_ these days, doing things like local $_ or local $^W. You'd really need some kind of pragma that would let you specify a given variable as "global only".

So you'd have to know ahead of time which variables you're likely to run into this problem with.

I think if you have your program/module grasped that well, you're unlikely to run into the problem in the first place.

Another reason not to do this is that things are hard enough to explain now, between use vars, our, my, and no strict 'vars'. I think adding another flavour might introduce more problems than it solves...
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Mike