I have only looked at perl internals briefly, but my guess is that it makes no difference, especially when you are using 'my' variables. The reason I say this is that from my limited knowledge of compilers, lexical variables usally are just used for the compiler which converts them to some type of internal represention that doesn't have anything to do with the actually name of the variable. I know that perl must keep the name around though, because you can do evals that include the lexical variables.
Even for package and global variables, I would imagine that it doesn't make much difference. The variable names would are probably prehashed, so the overhead of hashing a long variable name would only happen once at compile time.