When I use "save Image as" link on the Mozilla firefox, it saves the HTML for form instead of png code. (...)

if ( !$cgi->param ) {
### The form goes here
} else {
[download]

What does the form look like?  Maybe the problem is you're using POST to send the parameters, so the URL associated with the image doesn't hold the paramaters, in which case I suppose the browser won't resend them when you "Save Image As...", which in turn means the CGI will send out the initial form again...  (I'm presuming the browser will try to reload the image from the server, and not read it locally from cache, as dynamically generated content by default isn't cached.)

A couple of speculations... but anyhow, I would try GET instead.