As you can see from this JavaScript:

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['aspnetForm'];
if (!theForm) {
    theForm = document.aspnetForm;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>
[download]

you must set form __EVENTTARGET and __EVENTARGUMENT before submit it.

But why are you using HTML::Form and HTTP::Request? WWW::Mechanize will make all work for you.