That's an interesting question.

I'd guess for purposes of defining "fluent", a computer language could be considered fairly similar to a spoken language.  My native language is English, and I would say I'm a "fluent" speaker of English.  And, although I'm not fluent in any other language, I'm conversant in a few others.

Similarly in Perl, I feel I am more "fluent" than in most other computer languages, even though Perl wasn't my "first language".  And just as I occasionally need to lookup English words in a dictionary, I often need to lookup Perl functions, idioms, etc. in the documentation.

I guess, by extension to the analogy, one could liken cpan to specialized words in a spoken language -- if I needed to be able to hold a technical conversation with doctors, I'd need to understand a lot more medical terminology, whereas an in-depth conversation with quantum physicists might require a more specialized knowledge of the language of quantum physics.  Likewise, in Perl, there is a lot of "specialized" programming which I don't know intimately (if at all), but when I need to know it, I'll know where to go.

So, the more "fluent" I get, the more comfortable I'll feel using my language skills (in either Perl or English), but I'll never stop having to look things up.