Apple has shipped a somewhat untested security update, it seems. After installing it, my Catalyst apps stopped working in development. Trying to reinstall some modules through the CPAN shell brought further misery. Luckily, Miyagawa had already posted a solution to the CPAN problem.
In addition, it seems they have installed a broken Scalar::Utils, so after following his advice and installing IO manually to get a unbroken CPAN shell, I suggest running
cpan> force install Scalar::Util
To get a working weaken again. At least this fixes my stuff, but they might have broken more XS stuff. I’ll keep you posted if I discover anything else.