When attempting to install mod_perl2, I received an error code 2. Very unhelpful error message, but after some googling, I found this bug ticket which explains what you need to do.
Essentially, you need to forcibly uninstall perl5.12 (or perl5.10) and then re-install with the threads variant:
sudo port -f uninstall perl5.12 sudo port install perl5.12+threads
Then you can install mod_perl2 with:
sudo port install mod_perl2
Not a particularly elegant solution, but nonetheless gets the job done. Hopefully this issue will be fixed in the future.