In what way does it fail to work?

Please reduce one of your scripts to a small, self-contained script that exhibits the problem.

How do you fill %Form?

I would double check that the name of the hidden field ("hiddenfield") is actually the same between the HTML page submitting to the script and the script.

Assuming a standard CGI setup, the following could work, if your scripts use CGI.pm (and if they don't, you should start rewriting them to use CGI).

#!perl -w
use strict;
use CGI;
my $q = CGI->new();

print $q->header();

if ($q->param('hiddenfield') eq '') {
    print "<h1>Hidden field was not set</h1>";
} else {
    print "<h1>Hidden field was sent</h1>";
};
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