Checking the server logs, (and/or modifying the script to ensure that error returns from the system call are logged), should be able to confirm your suspicions regarding the cause.

It sounds as if this is the typical permissions problem associated with the default user id (typically IUSR_<machinename>) used by IIS. This account is accorded very restricted permissions, which typically will not allow it access to files below/outside the webroot of the server, which generally means that /tmp & /temp are off limits unless the default userid is modified to allow access--which you shouldn't do.

The "proper" solution would be to specify the location in which to create the temporary file using the DIR => '/webroot/some/path' parameter to the tempfile call.

Of course, if you are already doing this then I'm wrong, but that's what it sounds like.