The pertinent question is not so much "What are the best CPAN modules", but "What is the best CPAN module to perform some specific task".

The answer to the second question is very often, "All CPAN modules are best for some specific task". After all, the authors of modules spend the time to write and publish their modules because the modules provide a facility or way of solving a specific problem that didn't seem to be available when the modules were conceived.

Sure, there are plenty of unmaintained and even just plain broken modules around, and there are some that are clearly well written and solve their particular problems very well. But a list of the "best" is just barely usefull.

Of much more potential use is to change the focus slightly and look at "The most popular modules". Those are the ones that are used a lot, often because they are the best, but also because the solve problems that are frequently encountered.

So yes, I agree with your intent, but I'd shift the focus to "most popular", which is actually much easier to measure than "best" (see the What is quality? discussion). I'd focus less on problem domain and more on hints for usage of each module. Actually the two don't have to be exclusive, an index by popularity and another by problem domain solves the problem. Note that as soon as you start sorting stuff into slots though, bits drop through the cracks - good indexes are very hard to generate! (The popularity index is easy - but mutable, the purpose index is hard - but fixed.)