Hm, and how do you know if you need this or, say, that particular thing to complete your task or not?

This does require you have a basic understanding of the language you're dealing with. Using Perl as an example, one should have at least read through Programming Perl or something similar to get a feel for the language and the tools it provides. Staying current on advancements is also important and is what makes The Monastery such a great place :).

but please please at least explore as many things as you can just not to miss an opportunity.

I agree. Don't waste your time learning certain features in excessive detail when you could be gaining a greater understanding of programming in general, or learning how other languages compare to your current favorite.