As far as I can tell, Javascript::SpiderMonkey is simply a Javascript interpreter, but it is not integrated with WWW::Mechanize to execute the javascript that is found in HTML pages. It does not create a DOM (document object model), and unlike in a real browser, setting variables like document.location.href doesn't actually do anything (except set that variable.)

In your case, the page you get from the above url contains this <script> tag:

<script language="Javascript" src="./javascript/WebAdvisor_scripts.js"
+></script>
[download]

That .js file has the code which creates the user interface you are seeing when you get the page with a browser.