I unpacked the .tar.gz file, and sure enough, my file is there under a 'lib' folder, but the blib folder is not there.

Yes, the tarball needs to contain the "blib". I've always created the ppm tarball by running tar cvf My-Mod.tar blib followed by gzip --best My-Mod.tar as I have both 'tar' and 'gzip' utilities in my path.

There are, of course, other ways - such as randyk's PPM::Make.

Cheers,
Rob