Oh, dammit.

I didn't know it used the name of the tarball for anything. I knew that it extracted the tarball ('cause that's what it says in the docs) and I guess I just assumed that it used the names of the modules that it extracted for the namespaces it was going to modify.

What really bakes my noodle is

http://matrix.cpantesters.org/?dist=sendmail-pmilter%201.20_01

which appears to have collapsed everything to lower case. At a minimum that has to be a violation of the Principle Of Least Astonishment.

Thanks all, I'll remedy my little blunder pronto.