Quoting Win32API::Registry's documentation:

$hKey is the handle to a Registry key that can have hives loaded to it. This must be HKEY_LOCAL_MACHINE, HKEY_USERS, or a remote version of one of these from a call to RegConnectRegistry().

And SYSTEM isn't LMachine nor Users.

You probably want RegRestoreKey:

Reads in a hive file and copies its contents over an existing Registry tree.