The easiest way to reconfigure just one thing is to know its name and to change that from within cpan:

cpan> o conf foo bar
[download]

The conf command lists all configuration items:

cpan> o conf
[download]

If you don't find things there, another way is to edit CPAN/Config.pm and to change the values there.

As a somewhat related aside some advice:

If you're going to be heavily using stuff from CPAN you probably will save yourself headaches in the long run if you install a separate copy of perl yourself somewhere else (e.g. under /usr/local from source, or by using macports) and install things from CPAN into that copy instead. That way n months down the road when Apple pushes a software update that diddles the OS' perl install you're not spending time trying to figure out why module X suddenly is rolled back to the "latest" version from 3 months back.

Thank you for that suggestion, I do have macports installed on my Mac but never thought of install Perl from it. I'm sure macports will have a newer version than 5.8.9 anyways.