Hundreds, nay thousands. First started learning Perl some 15 years ago and I'm still discovering and figuring new things out. As an example, it's only been a few days since the first time I ever solved a complicated problem using a oneliner combining split(), map() and join() without peeking at the docs or someone elses code.

I don't really see it as an investment though, because learning Perl isn't something I do in order to achieve some goal. I see it more as practicing a form of art that only other geeks can truly appreciate.

Do it by numbers, do it for a living, do it to get better, because someone told you to, or just do it because you enjoy doing it.