An error occurred prior to the execution of the line that prints your headers. The server's web error log will reflect what error that is.

Possibilities include the server not having permission to execute the script, the script residing in a directory that isn't set up for CGI, the script has a dependancy on a library or module you created that hasn't been set up with proper permissions, or any of a number of other possibilities.

Another possibility is improper line endings (something that is OS dependant).

Basically I can only provide speculation, and may be way off base. If you don't check the server's error log you'll never be sure. Your webserver docs will tell you where to look for CGI errors. Maybe you don't even have CGI enabled on your server.