Something printing to STDOUT before the headers have been sent, and a blank line has also been sent to mark the end of the headers, or an incorrectly formatted header was sent. You can debug these issues by printing immediately print "Content-type: text/plain\n\n"; and looking at the output that is generated.

If you're using CGIwrap, you can use the debugging mode to see what content was generated, including the headers. suEXEC can also provide some debugging help.