how important is the presence of a community, real or, like perlmonks, virtual?

I'd say it's very important.

One important source of knowledge and insight is problem solving. That is (partly) what this site is about.

By being exposed to not only your own, but other people's problems, you pick up important, actual, real-world solutions. Canned experience. By being curious you can keep yourself informed about not only the things you do daily, but about the things you might be doing in the future. It widens your horizons. You get to hear smart people ask questions that you never would have asked yourself, making you (at least appear :) smarter in the process.

I find this tremendously beneficial to my own knowledge about general software development, to my computing street-smarts, and to my ability to have informed opinions about a wide range of subjects.

This doesn't even have to do with Perl, but with general concepts that you pick up by hanging in an environment (like Perl Monks) where they are used; MD5 hashes and what they are good for, error handling strategies, other programming paradigms, project management etc, etc. Things that may be difficult to come in contact with in day-to-day problem solving activities on your own.

/J