I don't see the current behaviour changing until someone completes a perl with a garbage collector instead of the current refcounting scheme.
The OP is saying that you can allocate a large string, let the variable go out of scope, and the memory is not freed and not reused. The memory allocated to the variable "sticks" to it even if you never use it again. (If I have this wrong, betterworld, please correct me.)
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.
I don't know what method perl uses to grow strings. The general method I recall from my CS classes was to double the size of a string when it grows out of its buffer and halve it when it shrinks to less than a quarter of the buffer size. Maybe someone more familiar with the internals can shed some light on why that wouldn't be a good design choice for Perl.