So strange... "easyjet.com"

[bzzt]

Hi,
I was trying to post with the following script to www.easyjet.com but failed
to get the proper response (the corresponding flights). Then (with no reason
at all) I gave value to a hidden field, and as a response I got an message
that this field is readonly and thus I cannot change the value BUT I got the
proper response.
Does anyone have a clue WHY does this happen?

#!/usr/local/ActivePerl-5.6/bin/perl -w
use warnings;
use strict;
use WWW::Mechanize;
my $agent = WWW::Mechanize->new();
$agent->agent_alias( 'Windows IE 6' );

$agent->get('http://www.easyjet.com/en/book/index.asp');
 die "Can't even get the home page: ", $agent->response->status_line
  unless $agent->success;
  my $form = $agent->form_number(1);
  $agent->field(orig => "ATH");
  $agent->field(dest => "LTN");
  $agent->field(oDay => "01");
  $agent->field(oMonYear => "092004");
  $agent->field(numOfAdults => "1");
  $agent->field(numOfPax => "0"); #This is the hidden field.
  $agent->submit();
  print $agent->content();