irclogger | Sinatra based irclogger.com | Command Line Interface library

BEGIN happens at compile time, and as you have observed correctly, this can be precompilation time.

I don't see any problem with that, as long as you don't assume compilation happens at script startup. Just like C++ templates are evaluated at compilation time, which is usually very different from execution time.

Also, should the use of BEGIN be encouraged (since values computed there are going to be stored in the precompilation cache and thus effectively eliminated from runtime) or discouraged

Everything should be encouraged for their appropriate use cases, and discouraged for everything else.

If you want to run something at program startup, use INIT, not BEGIN.

Is there some good use case for this?

Lots of meta programming can (and should) be done at compile time, for example creating a list of methods and attributes based on a fixed list of names. Doing that at every program startup would be a waste, and other parts of the program might need the complete type at compilation time.