Unintended side effects are unintended side effects... The problem has nothing to do with aliasing and everything to do with not localising!
But surely the soul of wisdom is to avoid unintended consequences.
With paranoia, one can only hope to avert bad consequences; with knowledge one can know you have.
Don't hope, know!
Again, this is utterly without consideration of effects on performance, but in general, shouldn't one prefer the avoidance of a coding technique that depends on local-izing everything in sight in favor of one that takes advantage of the lexical scope associated with every subroutine definition by using only lexical variables? By so doing, one may know one has averted bad consequences – at least outside the lexical scope of the subroutine one is examining at the moment. (BTW: Is it possible to define a subroutine without creating a unique lexical scope for it? Offhand, I can't see a way.)
Oh well... Perhaps not even USD0.02-worth on this one. Anyway, I have to go shovel some snow now.