mech and javascript modifying form post action

I am using mech to fill out a form. The problem is javascript is used to
alter the form before it is submitted. That is the html of the form
initially has <form method=post action=start.aspx>. When the user
clicks, javascript alters the form before submission to be <form
method=post action=somewhere_else.aspx?appended_stuff>. Since mech does
not understand javascript I cant simply click() or submit(). 
 
So, how do I fill out the form in mech and then change the target before
I use click() or submit? I can use standard perl to get somewhere_else
and appended_stuff, so that's not the problem. The problem is that I
don't know how to change mechs understanding of the form before
submission.
 
Any ideas?
 
 
Thanks,
Neville