You need to add the shebang line to your checklist. Apache (for one) will refuse to run a script without a valid shebang line. It probably belongs next to "What are the scripts permissions?".

On the subject of shebang lines, Devel::ptkdb is worth a mention for debugging. You can get this invoked directly from the shebang line

#!/usr/local/bin/perl -d:ptkdb
[download]

On a windows box, a ptkdb window will pop up when you access the url. On Unix, if you configure $ENV{DISPLAY} in apache you can get the ptkdb window to pop up in an X window.