... if (window.XMLHttpRequest) ...

Ok, why not use jquery here or another "library"?

There are examples of that on perlmonks, site:perlmonks.org psgi jquery , psgi jquery ...
->Simple example of CGI (Perl) + HTML + CSS + Javascript/jQuery + Ajax
->Using Perl, jQuery, and JSON for Web development
->CGI/Ajax example: lookup a value when a field is filled in

site:perlmonks.org plack jquery , plack jquery ...
->Re: How to store the value of a Javascript function as a Perl variable
->Mojolicious::Lite +and jQuery +AJAX + Mojo::Template