I'm a bit confused what you exacly mean, please correct me if i'm wrong. You have 2 forms in one page, and want the submit button to submit the proper form?
When using: 
<form action="javascript:alert('1')">
<input type=submit />
</form>

<form action="javascript:alert('2')">
<input type=submit />
</form>

[download]
It seems to work fine for me, i just can have a different action per form, and the button submits the form it is in.
update
Ah i see...
try: 
   $mech->submit_form(
        form_number => 1,
        fields      => {
            username    => 'mungo',
            password    => 'lost-and-alone',
        }
    );
    # as from $mech->submit_form( ... )
    # If button is not passed, then the submit() method is used instea
+d.

[download]
Its described here

In reply to Re: Clicking nameless buttons with WWW::Mechanize by jbrugger
in thread Clicking nameless buttons with WWW::Mechanize by bobf

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