Found a solution:

use Win32::GUI();
use Win32();
use Win32::GUI::Loft::Design();

...

# window exists called AddUser which has 8 radio
# buttons named: rb7, rb8, ... rb14
$win = $Win32::GUI::Loft::window{AddUser}
for (7..14) {
   my $rb = "rb$_";
   if ($win-> $rb-> Checked()) {
   ... found selected radio button ...
   };
   last;
};
[download]

Thanks for anyone who gave me advice.