I think a CPAN module should be the result of someone having solved a certain task by writing some code for the given problem. Even if that problem is very small, or very specific, or the solution is limited in scope or still incomplete, that code could potentially help ( or in the case of Acme modules, entertain) someone else, so it should be on CPAN.

Now, if the task is not solved yet, and the author is still looking for a good way to tackle the problem or is not even sure if he wants to finish this at all, it is IMHO better left outside of CPAN, for example in a Perl monk scratchpad.