PHP is great. Lots of extra skills is even better. Learn everything you find interesting and understand what a great leg-up the Perl community and code base is.

CGI is not any one technology. It's just the Common Gateway Interface; the interplay between server and client while passing more than simple requests. CGI can be in Lisp, Ruby, modperl, C, shell, Perl, whatever. The trick with Perl is that CGI.pm has become synonymous with CGI.

And probably the greatest, and concurrently the most terrible, part of Perl is that you don't have to be a great programmer to use/code pretty heady stuff. I'll quote/paraphrase dragonchild again: 90% of every Perl application is already written. Just cutting and pasting SYNOPSIS sections from CPAN POD you can write daemons, 2-way client/servers, graphs, robots, HTTP servers, RTF converters, XML feeds, Wikis, Excel parsers, and the list keeping rolling.