Here's a skeleton CGI::Application-inspired module. There are two run modes, one which is a fairly typical HTML::Template situation, and the other is the AJAX variety.

package YourCGIAppModule;
use strict;
use warnings;
use JSON;
use base 'CGI::Application';

sub cgiapp_init {
    my $self = shift;
    $self->param( 'json' => JSON->new );
}

sub setup {
    my $self = shift;

    $self->start_mode('main');
    $self->run_modes(
        'main'   => 'screen_main_page',
        'random' => 'ajax_get_random',
    );
}

sub screen_main_menu {
    my $self = shift;
    my $tmpl_obj = $self->load_tmpl(
        'default.tmpl',
        'loop_context_vars' => 1,
    );
    $tmpl_obj->param( 'title' => 'Main Menu' );
    return $tmpl_obj->output;
}

sub ajax_verify_browser {
    my $self = shift;

    return $self->param('json')->objToJson({
        'random' => rand
    });
}
[download]

I know this is really basic/simple, but it just doesn't have to be any more complicated than this. To get the data from JavaScript, you make the XMLHttpRequest object call and eval the returned text: