IMHO, I like the port-ish p5-Foo using whatever the CPAN distribution name would be. It's an "obvious" mapping ("Hrmm, Foo::Bar should be p5-Foo-Bar on github"), and the prefix distinguishes it as perl (and leaves room for the inevitable p6-Foo reimplementation).

(Which reminds me I should put my at-this-point abandonware modules up on github if I could ever remember what disk had my old CVSROOT on it . . . %)