Debian is a Linux software which is a free open-source software.
Mod_perl is an optional module for the Apache HTTP server. It embeds a Perl interpreter into the Apache server. In addition to allowing Apache modules to be written in the Perl programming language, it allows the Apache web server to be dynamically configured by Perl programs.
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages.
PHP is a general-purpose scripting language used for web development.