Ah, am I missing something else then? I'm not actually after the value of that key, rather, the handle that was used to open it.

From the Win32API::Registry documentation:

RegOpenKeyEx( $hKey, $sSubKey, $uOptions, $uAccess, $ohSubKey )

$ohSubKey will be set to the handle to be used to access the new subkey if the call succeeds.

(It's also possible there are some typos in the POD on CPAN...)