I don't know what you're smoking, but it must be the same stuff that Schwern's been taking. Let me cite from the documentation on --installbase:

Note that this is different from how MakeMaker's PREFIX parameter works. PREFIX tries to create a mini-replica of a site-style installation under the directory you specify, which is not always possible (and the results are not always pretty in this case). install_base just gives you a default layout under the directory you specify, which may have little to do with the installdirs=site layout.

If this is the "workaround", it is very well hidden that this can somehow reach a solution compatible to PREFIX=.

As an aside, I found this information at the very end of the Module::Build documentation. This information should be at the top, so a "consumer" user finds it without problems. Maybe there should be a separate Module::Build::API document that documents the "producer" API, which far less people care about. I guess you'll think of this as a feature request, or a bug report, and it should thus go onto RT, or on some list. But I don't care about M::B enough - I only fear that too many people will use it.