There's GNU APL as a free APL implementation. As for learning APL... It's essentially learning what all its symbols do (there are some references for free online from the vendors of paid APLs, just avoid reading about their extensions,) even Wikipedia's page about APL works as a fast reference. I just use it to play around with numbers (yes, I also wrote a Mandelbrot set with it, a one liner that fits in a tweet,) and I'm trying to enhance a little its usability with a iPython-like in-browser "REPL" specially suited for my uses.