I don't see what garbage collection has to do with this. The strings in question don't have any references to them, so the reference counter shouldn't have any problem knowing that they're not in use.
Reference counting has everything to do with it, since it means that the only time perl can free the memory is when the last reference to the scalar goes out of scope. All without knowing if that scalar is every going to be reused.
That means it either has to keep it there always, or free it always (or do some kind of heuristic, which should usually mean keep it, since allocating memory is expensive, and if you're using a large string now, chances are, you'll be using a large string again some time soon).
What perl currently cannot do, is free "old, unused" scalars when it's running out of memory. It has to decide when the scalar is going out of scope. allocating and freeing each scalar every time that happens would probably slow down the interpreter a lot.