This shirt says #!/usr/bin/perl on the front. That is known as the perl shebang. It is put at the top of a perl script, and it tells the web server (like Apache) where to find the perl executable. This way it knows how to run the code following the shebang. Perl is the scripting language I use to run this website. I originally learned perl at the turn of the century using CGI Programming 101 (First Edition). On the back of the shirt you will see a quote from Larry Wall, the inventor of perl. The source of the code is here (tidied up here) and you can execute it here.