Thank you!

I have followed your indications and I have found in  http://search.cpan.org/~gaas/HTML-Form-6.00/lib/HTML/Form.pm that buttons have always the same name. After this enlightenment, I have read  http://www.perlmonks.org/?node_id=547270 and I have found the solution.

I only need to write:

 $mech->set_fields('%%RBGROUP16843009'=>'wt_2000-selector[3]');

Thanks again.