A bit off topic for a Perl forum, however, IIS generally runs (anonymously anyway) with the IUSR_machinename local user.

You will need to check two places for permissions: firstly open the web site in the IIS admin tool and make sure the permissions are set to allow write access. Then open the directory in the file system where you're trying to write to and ensure that the IUSR_machinename account has write access to the directory. It may also need read/write access higher up the directory tree as well depending on your configuration.

You could also turn auditing on those directories and check the security event log to see exactly where it's being denied if this doesn't work.

HTH,
Loki