Before trying to install anything, you should try the following from the command line:

perl -MLWP::UserAgent -e 'print "Good!\n"'

In Ubuntu, there is a version of Perl tightly bound to the OS, and it includes LWP::UserAgent. I'm not certain why, but I believe it is part of the package management system.

One of the side effects of this is that Ubuntu can be a pain if you need to install modules - before invoking CPAN, I highly recommend you check in apt/aptitude/package management to see if there's an official distribution version of the package you want to install. In order to avoid some real frustrations when there isn't an official distro, I either use a local, user-specific copy of perl or at least local module installs so I don't have to deal with packages installed from multiple sources.

Update: This thread from the Ubuntu forum seems on-topic.