#!/usr/bin/perl

use strict;
use warnings;
use CGI;
use Scalar::Util;

my $cgi = CGI->new;
my %params = $cgi->Vars();

if( defined $params{verstecktes_feld} and looks_like_number( $params{verstecktes_feld} ) ){
    print "Zahl im versteckten Feld\n";
}