When I first began developing in Perl, I knew only the most rudimentary Unix commands (ls, cd, chmod, etc.). I got by for quite a while using CGI.pm and the DBI with that level of knowledge, largely because Perl can do most of the things that need doing with the operating system (that, and the webserver admins didn't really want me on their box anyway). Over time, though, as mentioned in other responses, I was exposed to various flavors of Unix and remedied that ignorance. In hiring, I would prefer someone who had at least a base level of Unix knowledge over someone who only knew Perl.

With regard to learning Perl, Unix knowledge is certainly not a prerequisite, although one might find that knowledge in Unix would tend to accelerate the process of learning Perl, and vice-versa.