In context:

In Windows, the environment is passed down by the parent process on creation of the child process. In this case, the parent process is the web server. %ENV simply accesses this environment and works without knowing the meaning of any variables in the environment. Perl simply works with whatever the web server provides. Any differences in the environments are caused by differences in the web servers and/or their configurations. That (not in Perl) is where you should be focusing your efforts.

By the way, AUTH_USER and HTTP_PROXY_USER are extentions to (i.e. not part of) the CGI standard.