Iverson's later language, J, is also open source. It doesn't require a special keyboard, has top notch documentation and online support resources and is open source under GPL3. It might or might not be a better place to get started than APL depending on a person's goals.
It's not really open source. Only the "core." The development environment is excluded.
(Looked into it recently and I was disappointed.)
Between the command line, scripts and Emacs the J IDE isn't really a big deal for me. Because J was a closed source commercial product and dates to the 1980's their may be copyright licensing issues tied into the code base. It's hard to release existing projects into open source.