is probably the most powerful and efficient module both in terms of memory usage and speed, but it has its learning curve.
On the other hand, there are several modules on CPAN allowing one to access packed strings as regular arrays as for instance Packed::Array, Tie::Array::PackedC or my own Tie::Array::Packed.
Those modules are memory efficient, but relatively slow due to the tie interface overhead. Well, actually, for instance, Tie::Array::Packed which is written in fast XS, would probably be on par with using perl builtins (i.e. substr, pack, unpack, etc.) to access a packed string. Probably it depends of the specific operations being performed.