<form>
<input type="checkbox" name="passenger" value="somevalue" />
<input type="submit" name=".submit" value="OK">
</form>
[download]

The following example (tick.pl) demonstrates the checkbox ticking.

use strict;
use warnings;
use WWW::Mechanize;

# set some dummy vars
my $target_field = 'passenger';
my $target_value = 'somevalue';

# get the URL and check
my $url = shift or die "Need URL, pal!\n";
my $mech = WWW::Mechanize->new;
$mech->get($url);
$mech->success or die $mech->response->status_line, "\n";

# select the form
$mech->form_number(1);

# check original value
my $set_value = $mech->value($target_field);
$set_value = 'undef' unless defined $set_value;
print "[1] value for $target_field: ($set_value)\n";

# tick the checkbox
$mech->tick($target_field, $target_value);
$set_value = $mech->value($target_field);
$set_value = 'undef' unless defined $set_value;
print "[2] value for $target_field: ($set_value)\n";
[download]

$ perl tick.pl http://localhost/form.html
[1] value for passenger: (undef)
[2] value for passenger: (somevalue)
[download]

I'm not an HTML guru, but I don't see anything wrong with the HTML you posted. Maybe you should show us the Perl you've written along with an explanation of what you expect it to do and how it is not meeting those expectations (i.e., what does it do).