The problem is that disabled form fields do not get submitted when the form is submitted, they are completely ignored by browsers, so HTML::Form is also not submitting them. You already have a method call to find a field and force a value:

$m->form_name("form1")->find_input("MakeID")->force_value("32");

I looked at the HTML::Form documentation and there is a method called disabled() that you can use to toggle the "disabled" flag on a form field. So I added this to your code:

$m->form_name('form1')->find_input('MakeID')->disabled(0);

And then submitted it using mechanize, and I got back a list of volkswagons.

The tricky part is that although the field is disabled by default, somewhere along the way, their javascript enables it. To use mechanize on javascript-enabled sites, you really have to look at what's being submitted to the server when you press submit on your browser, then you have to use mechanize to emulate that.