I don't have any specific answers to your questions, but I do have some relevant suggestions that might help:
I'm not sure why you're setting INSTALL_BASE. Its not necessary when making arbitrary perl installs anyways. Adding the perl you want to use in that session to PATH should be enough.
Its likely that all of these installs you've fiddled with are using the same directory (~/.cpan) to stage and build perl modules. That might be trouble.
You can see how I manage hand built perl installs at Re: 2nd Perl installation on Mac OSX.