Perl is simply today's most popular choice for CGI applications. It isn't really that important. If you're serious about programming, you should be at home in a range of languages. You'll certainly want at least one scripting language such as Perl, Python or Tcl. My book is one of the few that tackles two languages at the same time.
Language is just a tool to do so.
It depends on your server installation. These types of filenames are commonly used conventions - no more. If you are running your own server, read the manual. If you're on ISP or other rented webspace, check their webpages for information or FAQs. As a last resort, ask the server administrator.