Hi,

Normally it's because you have sended something prior. Take a look to a nice extension for Firefox called LiveHeaders to see what you getting from that CGI.

For example on module:://HTML::Mason you have a method to flush the output buffer, and another to send an HTML code

$m->clear;
$m->abort(404);
[download]

$m->clear_and_abort(404);
[download]

Regards,