Was the bug in Perl or its libraries, or in the code written in Perl? There are many valid criticisms of Perl, but I've never heard of the language itself described as insecure, and especially not memory-unsafe. I don't know how I'd write a use-after-free or stack smash in Perl if I were forced to.

Yep, there are bad bugs for example in mod_perl which is written in C and takes the interpreted Perl code and runs it in the Apache context. I think this is what the OP "heard about". But that is not the fault of Perl itself.