##

$mech = Test::WWW::Mechanize->new();
...
$mech->submit_form( with_fields => { ..., 'birthdate' => ['01',0] });

# or

$mech->set_fields({ ..., 'birthdate' => ['01',0] });

##

##

$mech->field('birthdate', ['01'], 0);
$mech->field('birthdate', ['17'], 1);