Your username shouldn't really have rights to install modules into the central library repository, so it looks like this server is configured correctly (and I'd be a bit worried about the ones where you can install modules like this!)

You either need root permissions to install modules into the central library (I like to use 'sudo' for this) or you can install modules into a local library and use the appropriate "use lib" line.

Update: Hmm... re-reading your post I see there's a specific problem with /tmp/.cpan. Perhaps someone else has been installing modules on that server and has left that directory with problematic permissions. I suggest either removing or renaming that directory (you'll probably need to be root to do that) and trying again.