Posting to xmlrpc.php return 412 error code 'Precondition Failed'

I tried this perl code:

use Frontier::Client;
$server = Frontier::Client->new(url => 'http://www.smartango.com/xmlrpc.php');
#$server = Frontier::Client->new(url => 'http://betty.userland.com/RPC2');
$list = $server->call('system.listMethods');
print "$list\n";

That works with betty.userland.com/RPC2 but not with the drupal instance in www.smartango.com .
I have the same problem with a C client I am developing, does it need some extra header setted
someway?