I love my REPL and I use it all the time.

I've added all sorts of things, like uses for my favorite utilities (eg.List::Util); aliases for stuff (!....;; => system 1, ...); etc. As I get new ideas, I try them out. Anything that interferes, I discard. It really is the single most used Perl tool I have.

But I wouldn't want to try and formalise it; and document it; and maintain it to the standard that would be required for me to consider releasing it for others to use. At that point, it would become a burden on my time, rather than the productivity tool it currently is.