More info. Here's the notification I get back when I turn on e-mailed output:
/usr/bin/ruby: Permission denied -- /home/myname/mydomain.com/cgi/test.rb (LoadError)
Which permission don't I have? To run ruby, or to load test.rb? The permissions bits for the cgi folder and the script are both 711.
When I launch test.rb manually, it works fine.