find_all_inputs is documented to work on the current form.

It wasn't hard to find methods that set the current form: form_number, form_name and form_with_fields.

Note that form_name returns a form, while find_all_inputs needs to be called on a Mechanize object.

$mech->form_name('account');
my @c_inputs = $mech->find_all_inputs();
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Untested. Just going by the docs.