The web server is probably running as a different user than you expect, so the CGI probably lacks user or group permissions to write.

Besides that, your argument parsing has several flaws:

• It doesn't account for the semicolon, which is the recommended separator for name/value pairs
• It doesn't check the content length, so you leave yourself open for a denial of service attack
• It only takes arguments from POSTs, not GETs, which may or may not be an issue

The two error messages you're seeing are really from the same issue. The die happens before anything is printed to the browser, so the web server sends back content saying "Sorry, the script died before it could send anything."