I've been coding in perl for over 20 years now (I wrote C for about 10 years before that), and I'm always looking stuff up, because the man pages are good enough (and I've gotten good enough at reading man pages) that I don't go out of my way to memorize stuff. (If I happen to remember how a given module or function works, that's fine, but not essential.)

As for just sitting down and writing code, I pretty much always start every new script file by writing the man page for it. That's the only way I can be sure that I know what I'm doing, and whenever I haven't done that, as often as not I get confused before I get very far.