Question:
Why do I see the cart.pl script code when I point my browser at it? Why doesn't it execute instead of being served as a plain text document?

Answer:
Your host is serving it as plain text. Try renaming it to "cart.cgi" and see if it executes then. If not, send a link to the script to your host and ask them to set permissions on it so that it will execute and not serve as plain text.

If you are on a Windows server, ask your host and ask them to go to mappings and add this:
.pl c:\Perl\bin\Perl.exe %s %s GET,POST check check