TestManager has an API which allows you to script your tests using the language of your choice. Unfortunately, that is the extent of the API. It doesn't give you access to the underlying database, or to the other internal operations of the TestManager application. (This is in stark contrast to the APIs for ClearCase and ClearQuest, for example.) The database is stored as flat files (actually an XML dialect) on disk, so you should be able to poke around in the database yourself and figure out how to manipulate them. One caveat: make sure TestManager itself is not running while you do this; it is extremely non-robust in the face of concurrent access by other processes. This is (partly) because the TestManager application caches the database (from the XML files) in an in-memory database. Whether or not there is a Perl module for this, I don't know.

At least, that's my personal experience. Perhaps the situation isn't as bleak as I paint it. Have a look at Demystifying the IBM Rational TestManager API - an article on the IBM Rational developerWorks website.