Just got home and tried to reproduce this on my private machine, it worked just fine.
So I guess this is something on my work machine, maybe version control related or something.
Still strange this happened while running emacs -Q.
I can try your file tomorrow, but my guess is if I drop this into one of my project directories it won't work.
I'll try it outside of version controlled directories, too, and provide an example file.
But if it turns out this is somehow related to version control or something else in my project tree, this will be hard to debug and might not even be directly related to cperl-mode at all :(