Perl
mod_perl and Apache
Here is a guide to installing mod_perl on Apache (out of the box, Slackware 12.0 runs perl as a cgi script): Mod_perl on Slackware 12.0
Embedded Perl
Embedded Perl is a way you can put a chunk of perl code in the middle of an html file, and apache will run the perl code and put it's results at that point in the html before it sends the page to the remote browser. It's older and a bit arcane, most people would use mod_perl or cgi perl these days. Embedded Perl on Slackware 12.0