I would argue that it would be the borrowers of the books, not the librarians, doing all the checking - and every reader of a book CAN send a note to the publisher regarding typos, etc - and just like CPAN, if a publisher/author is no longer reachable, the work never gets updated unless someone else takes over maintenance. I initially thought that broke the parallel between modules and books, but then I realized there are textbooks, encyclopedias, etc, that are maintained by more than just one person, so the analogy still holds.

As far as I know, there isn't any code review by the keepers of CPAN on the modules themselves. Automated testing maybe, but not actual review. Maybe I'm wrong on that, but it seems unlikely given the number of submissions they'd have to review.

I've probably pushed this farther than anyone is interested in... so I'll stop now. :)