Retrieving that URL gives you a 403 Forbidden error, with an error page that points you at http://www.google.com/terms_of_service.html . This is in place because Google bars automated querying of its site.
LWP::Simple's get function doesn't have a way for you to see the return codes, so you wouldn't have seen that this causes an error. (If you want such information, use LWP::UserAgent instead.) Instead, the function just returns an empty string, as you saw.