If you've ever built Perl from scratch, you probably know how much faster it can be to make and test in parallel. On the other hand, if you use perlbrew on a multi-core processor, you probably already figured out that it wasn't using all your processors.
I was very pleased to discover an undocumented '-j' option in perlbrew 0.17 that switches on parallel make:
$ perlbrew -j 5 install perl-5.12.3
Currently, this only runs make in parallel, but I've submitted a patch to make it switch on parallel testing as well (for recent Perl's that support it). The patch also documents the option.
I hope the new version will be out soon, but if you have 0.17, you can already start using '-j' for a small speed boost.