foreach my $expect (@expected) {
       my $received = $q->param($expect);
       unless (defined $received) {
           $error .= "Didn't get $expect parameter!\n";
       }
       push @request, $received;
   }
if($error ne "") {
       # Handle the error here, reprinting the form,
       # pre-filled with what they've already sent.
       # Also tell them about all the problems:
       print $error;
}