The short answer is that access to hash elements (almost) does not depend on the number of elements in the hash (it is said to be O(1)), so long as you don't saturate the size of your available memory.

One qualification however: there can be cases where a hash become pathological, but these are extremely rare (if ever) with the current implementations of hashes in Perl. For practical purposes, the limit is really the quantity of available RAM, taking into account the fact that the hash structure takes quite a bit more memory space than the sheer amount of data you store in it.