GENERAL

Perl

Perl is a high-level programming language with an eclectic heritage written by Larry Wall and a cast of thousands. It derives from the ubiquitous C programming language and to a lesser extent from sed, awk, the Unix shell, and at least a dozen other tools and languages. Perl's process, file, and text manipulation facilities make it particularly well-suited for tasks involving quick prototyping, system utilities, software tools, system management tasks, database access, graphical programming, networking, and world wide web programming. These strengths make it especially popular with system administrators and CGI script authors, but mathematicians, geneticists, journalists, and even managers also use Perl. Maybe you should, too.
For more information about Perl visit Perl.com and cpan.org.

CGI

CGI is not a language. It's a simple protocol that can be used to communicate between Web forms and your program. A CGI script can be written in any language that can read STDIN, write to STDOUT, and read environment variables, i.e. virtually any programming language, including C, Perl, or even shell scripting.
Many Recourses are avaliable for CGI we recommend visiting cgidir.com and w3.org.

If you use UNIX a lot you should already know what sendmail is, but like most us you most likely use a Windows OS and have no idea what sendmail is.
Sendmail is a program that is used by UNIX servers to send E-mail. If you have ever sent an E-mail message, you have either used sendmail or a windows version of it. There are many discussion boards available at sendmail.org

SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated on the server while the pages are being served. They let you add dynamically generated content to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology.
For more information on SSI visit apache.org