Not really a comment on the code, but the ideas it triggered. I know perl has a way to create tar files (even find2perl can create tar files), and Archive::Tar can read as well as write files.
Did you need to create temporary files because cksum couldn't take input from stdin?
It would be nice if we could treat tar files as file system - but that would require special modules loaded into linux, although maybe Perl could virtualize it as a file system.