Is that error showing up in your browser, or in the server's error log?

If the script fails to compile properly, or if it produces a warning before commensing its run, it will output text that will cause a corrupted header.

What happens if you run the script from the command line? Often you can do that, particularly if the script was written with CGI.pm. I've found that doing so will usually give you a better idea of the problem. In particular, you'll be able to determine more easily if the message is a compiletime error, warning, or a runtime issue.