Sorry, I can't directly point you to a CPAN module, but have you considered ducking the issue by revising the text slightly? Translate "Richard's Library" into "The Library" (under the Heading "Richard") or "The Library of Richard".
I can do that in some places, but not all of them. I'm not the siteowner, and so somewhat bound to the framework made by someone else. This framework was previously used to translate the site in Russian, Norwegian and French, so if I mess to much with the framework, I migth end up breaking the other-language versions of the site.
I agree with the idea that it would be better to have a different structure, such as Dave :: Library :: Fiction or somesuch. That's much more portable across languages - and unless you're quite sure that the Russian, Norwegian and French translators did a perfect job then the site owner may well be interested in rethinking his/her posessives.
If you absolutely positively have to keep Willem's Library and so on, you might want to store the posessive form in your database and only change it when the name changes. My guess is that you'll have a monster bit of code - if a very wise monk steps forth maybe a monster regexp - and you may not want to run it too often.
I'm not an expert in these things but you might also want to look into lexemes/morphemes, though I honestly have no idea how applicable they are to proper names.
Good luck! It's an interesting challenge, and I hope you're being paid by the hour :-)