• Declaration happens at compile time which should be neglectable.
• Assignment should be the same. (Or even faster°)
• The only overhead could be destruction or resetting at end of scope.

Your example with a for loop is a bit unfortunate, because aliasing is complicating things considerably.

Together with various optimizations performance gains or loses should be unpredictable, especially between different versions of Perl.

So benchmark it, I don't think it's worth the effort.

°) BTW, for many years it was commonplace that private variables are faster, not sure if accessing package variables has been optimized in the mean time. ...