FWIW, I couldn't get your version of the script nor the RenameUser.pl that comes with the Win32::AdminMisc module to do anything either.

The total absence of any mention of the RenameUser() api in either the module itself or the pod that comes with the module make it very difficult to track down the likely cause. The function exists in the dll (apparently, as perl doesn't complain when you try to use it) but when I tried running the test script that came with the module I got

P:\site\lib\Win32\AdminMisc>renameUser
Attempting to rename the user account 'fred' to 'derf'
Could not rename the user 'fred' to 'derf': The operation completed su
+ccessfully.

Finished.
[download]

Which is somewhat confusing:)

The best suggestion I can think of is that you try contacting Dave Roth via his web site or CPAN id.