When I make forms of all kinds (which is all the time since I only program under CGI), I test required fields using if/else iterations. Ie.

if (param('submit'))
{
   my $name    = param('name');
   my $email   = param('email');
   my $message = param('message');

     if($name ne "")
     {
       print "Good, you have a name!\n";
     } else {
       print "Hey buddy, I know you have a name!\n";
     }

     if($email ne "")
     {
       print "Alright, I know your email addy\n";
     } else {
       print "No email addy from you? No email to me!\n";
     }

     if($message ne "")
     {
       print "Thanks for your message\n";
     } else {
       print "Excuse me?  I didn't hear you say anything!!\n";
     }
[download]

You are testing each field against "" (or nothing). If you want, you can add an exit; command after it finds a blank field like I usually do.

Hope this helps.