mechanize requires you to select the form before you can set the form
fields. This can be done with a variety of functions such as
$mech->form_name( $name [, %options] )
$mech->form_id( $id [, %options] )
$mech->form_number( $number [, %options] )
$mech->form_with_fields( [$options], @fields )
Taken from here.
If this doesn't work, you can always run some custom code in the page
context to change the fields. For radio boxes this would look like
[name='radioButtonName']")[arguments].checked = true;
Although, this only changes the property of the radio button and in most
cases this is enough. But sometimes there are event handlers on the DOM
elements which will not be triggered in this case.
An alternative would be to actually use
[,$x ,$y] ) to click on the radio button and trigger all necessary