.pl is the usual file extension for Perl [perl.com], the granddaddy of scripting languages used on the web. Hugely popular and powerful.
'CGI' stands for Common Gateway Interface. It's the protocol by which web servers pass information to other programs - usually scripting languages, but can be C(++) or whatever. It's the main method of creating dynamic sites.
As to Cpanel 4 - sorry I don't know. Sounds like it's the control panel software your host uses to let you access your account's features.
Hope this helps