Well, it was installed when I ran sensors-detect, but it seems Perl is no longer present after rebooting. I just ran `installpkg /boot/packages/perl-5.18.1-x86_64-1.txz` again, uninstalled and re-installed the plugin, and now the aforementioned Detect button behaves as expected.
Other than putting the Perl archive at /boot/packages/perl-5.18.1-x86_64-1.txz, is there something else that I need to do to ensure Perl is always available after reboots?
Side note: I wonder if this has something to do with the order of steps, specifically when the plugin is installed. Unless I'm missing something, there's no mention in these docs at which point the plugin should be installed: https://lime-technology.com/wiki/index.php/Setting_up_CPU_and_board_temperature_sensing
Perl is one of the packages you can install from the NerdPack plugin.