I don't have a Windows box to test this on, but based on the docs for CGI::Session::File, I believe cees was right in that /tmp doesn't exist on a Windows machine; {Directory => '/path'} is looking for a filesystem directory, not a subdirectory on your webserver (as you described).

Have you tried changing it to {Directory => 'C:\temp'} or {Directory => 'c:\apache\cgi-bin\tmp'} instead?