Just finishing up this thread in case someone else runs into this same issue. It turns out that WWW::Mechanize allows you to build and use your own HTTP::Request object through the use of the $a->request( $request ); method. This bypasses the need for HTML::Form, and allows you to enter any information into the form fields.

Here is a code snippet that shows how it can work:

use Test::More qw(no_plan);
use WWW::Mechanize;
use HTTP::Request::Common;

my $url = 'http://starting.url/';

my $a = WWW::Mechanize->new();
isa_ok( $a, "WWW::Mechanize" ) or die;
$a->get( $url );
is( $a->status, 200, 'Fetched OK' );

my $form = $a->form_name('details');
ok($form, 'Details form');
my $action = $form->action();

# build a request object
my $request = HTTP::Request::Common::POST($action, [
                       a_radio_button => 'invalid_value',
]);
# Add any cookies into the request object
$a->cookie_jar->add_cookie_header( $request );

# make the request
$a->request( $request );
is( $a->status, 200, 'Fetched OK' );
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Thanks for the help and suggestions

- Cees