What you want to do is use CGI.pm to check to see if you have parameters set, and if not display your initial html code. Make sure you don't put an action in your form, though. That way the CGI.pm module will redirect the information to the calling URL.

Pseudo code:

if (param()){
    do your error checking here
    
    exit; # exit the cgi script if all is correct
    }

HTML code here
...
...
[download]

Bascially what this does, it to see if you have called the script with any parameters. If not, then it defaults to the displaying of the HTML code. If you have passed the parameters, then you can see which ones need to be processed and/or corrected and go from there.

I believe there are examples of this in the CGI docs, plus there are a few examples of it on the web.

HTH!