You have to do it with a combination of Perl and Javascript. Something like this would do the trick in Javascript:

function submit_on_change() {
     document.myform.submit();
}
[download]

You have to change "myform" to match the name in your form element (form name="myform" it can be whatever you want). Then call the Javascript function in your onChange on the select menu:

onChange="submit_on_change()"
[download]

Hope that helps.