I'd suggest one of two options:

Do as ichimunki suggested and release the code as a separate module. The suggestion of MP3::OpenNap was a good one. This gives the end user the choice of which module to use.

Release a patch.

Whatever you do, don't release that submodule if what you're really doing is rewriting large portions of the internals. Submodules (IMHO) should only use the standard documented interface of the module, inless you're explictly putting the module in ISA, in which case all bets are off (and even then, I'd still recommend using as much of the documented interface as possible).