I think it is perfectly acceptable to put a module on CPAN that includes no automated installation and just tells you to copy the relevant file(s) to the relevant locations. There have been, and very probably still are, such modules on CPAN.

Besides, using Module::Build only adds a prerequisite to installing your module. Many modules require that you first download other modules before building and installing. How is this different except that the CPAN module doesn't support it. (gad, who really uses CPAN for anything other than searching and downloading modules anyway, doesn't anyone look at sources before running make anymore?)