I can't answer your question, but maybe I can offer you alternatives

• Don't install CPAN packages directly, but build binary packages for your linux/unix system. For Debian you can use dh-make-perl, there are other tools for other distributions. The version in Debian Lenny nicely follows dependencies.
• Make the locations to which cpan installs stuff writable for a user named cpan, and run cpan as that user. (Heard from JavaFan on the CB the other day).