getpwuid and getgrgid are unimplemented by ActivePerl for Windows. Instead I used Win32::LoginName and I see my personal user ID. This agrees with several variables found in %ENV such as
AUTH_USER
LOGON_USER
REMOTE_USER
except those variables also include the domain name ("DOMAIN\userid").
The USERNAME environment variable is showing as the server name ("MYSERVER$") and I think perhaps this is a remnant of the IIS process before it invoked the CGI module and impersonated the remote user making the request.

Maybe there's something else in a Win32 module that I can use to diagnose the situation. For example, I'll use Win32::File to check whether the folder has the readonly attribute set.