I've used WWW::Mechanize but never the FormFiller part so this may or may not help you but I noticed in your code that you never use the name of the form. When I've used WWW::Mechanize, I've had to put the name of the form I posted to so my assumption is that you have to do that too, otherwise it won't know which form to post to.

This is a partial example of what I did to pull names and addresses from yellowpages.com:

use WWW::Mechanize;

my ($name, $city, $state);

my $url      = 'http://www.yellowpages.com/Index.aspx';
my $mech     = WWW::Mechanize->new(autocheck => 1);

$mech->get( $url );
$mech->form_name("__aspnetForm");
$mech->field("Mainhomectrl2:BasicForm1:businessname", $name);
$mech->field("Mainhomectrl2:BasicForm1:city", $city);
$mech->field("Mainhomectrl2:BasicForm1:state", $state);
$mech->click("Mainhomectrl2:BasicForm1:searchbtn");
[download]

I think you need to add $agent->form_name("post") to your code. I just tried it and it didn't throw an error when I added that to your code.

HTH!