I am no expert, but here is my hypothesis. (And I will gladly admit where and when I am wrong, provided evidence to correct myself.)

I think that communities take a lot of their characteristics from their founders and early members. The perl community was lucky enough to have a number of early members who not only set a tone that is (mostly) friendly and helpful to those willing to help themselves, but also thought consciously about the community that (they hoped) would develop, and as such set it on a path that has led to where it is today.

(At least that is my take on it. I look forward to the comments of others on this.)