First off, are you using the system Perl or have you installed your own? You generally don't want to install things in the system Perl-- the system depends on the Perl that came with it being the way it expects it to be. I'm not up to Lion yet, Snow Leopard shipped with 2 or 3 versions, depending on whether you have a 32 or 64 bit machine: 5.8.9 (32 bit), 5.10.0 (32 bit) and 5.10.0 (64 bit). I'm still on an old 32 bit machine, and managed to install a lot of modules into the 5.10.0 version without breaking anything (I vaguely recall that the system depends more on the 5.8.9 build, but I could be wrong there), but did have some problems with compatibility with some modules not behaving, even though I had all the pre-reqs installed
Your best bet is to install your own version of Perl and add your modules to that. There used to be a lot of pages floating around with instructions as to the best way to do it, but now Perlbrew seems to be the most convenient. It lets you install multiple versions of Perl and switch easily between them. It also comes with cpanp, which works quite well
If you want to go ahead and add modules to your existing install, you might (or might not) have better luck running CPAN in shell mode-- I've pretty much always done it that way and it generally will prompt for pre-reqs as it goes. You can get there with sudo perl -MCPAN -e shell and then install away from there.