A user-editable userland is a pretty neat idea, and if your language of choice is Perl, why not? Plan 9 has the pretty neat 'src' command that immediately opens the source file (or main entry point) of a given command.
Given something like file system overlays, individual users forking their own command versions would be a pretty neat idea, albeit it might turn out to be a bit chaotic. Although not much more than e.g. a highly-customized shell or emacs environment.