any other suggestions about how I can get the test to pass, apart from bypassing the test altogther?

Figure out why you're getting permission denied and fix it. How? Start by verifying that everything is the way it should be, from permissions, to the enviroment, to the modules you're loading and the perl you're using.

Also run the test manually (path/to/perl -Mblib t/robot/rules-dbm.t ).

I'm guessing that the DBM facility is a vital part of LWP::UserAgent, otherwise I'd just comment out the tests and get on with it.

You're guessing wrong. Unless you plan on using WWW::RobotRules::AnyDBM_File, just move on (and no, you don't need to comment out the tests to do that). And even if the DBM facility was a vital part, its a permissions error, I doubt it has anything to do specifically with WWW::RobotRules::AnyDBM_Files.