windows Git.pm setting GIT_DIR?

We are attempting to make a perl application using Git.pm run on windows.  One
of the things we do is initialize 

        Git->repository(Directory => $some_other_directory);

This works fine on unix, but on windows the $some_other_directory seems to be
ignored.  This seems to be because _cmd_exec or its equivalent to set GIT_DIR
and chdir does not exist in the windows code-path in _command_common_pipe.  Am
I missing something that should make this work, or is this just not supported?
(Sorry, I know little about windows and trying to interpret problems others
are having.)