By pure-perl, I mean the code written by the developer, not necessarily any modules being used.

There's a whole bunch of audio related modules on CPAN. Even if you don't want to use any modules for some strange reason(?) they are worth taking a look at to see how other's have approached the task.

In particular, the docs for Audio::Wav state that the module is written entirely in Perl. I don't have much experience manipulating audio data though, so I could be completely off the mark here :).