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<form method="POST" id="loginForm"
  action="https://www.pharmgkb.org/cas/login?
...
    </tr>
  </table>
</form>

print Dumper( $mech_obj->current_form() );

use strict;
use warnings;
...
$mech_obj->form_number( 2 );
$mech_obj->set_fields( 'username' => $username,
                       'password' => $password );