More information would be great....

The filename of the $idfile is determined by the program, and references to a file almost local to the script (one directory below). Anyway, it is relative local path.

Update
The file opens for reading without problems. The halt occurs on trying to open the file for writing. Using absolute paths does not make any effect.

Have you checked the security permissions for this file? IIS will use the IUSR_<servername> account when your CGI script is run. Normally the IUSR_ account only has read access to files.

Try setting the file security to Everyone / Full Control to see if that helps. You can change the security again later to remove the Everyone group and use the IUSR_ account explicitly.

the command does not produce an error message

Unfortunately IIS doesn't capture errors and save them in an error log like Apache, so this is one reason why you would be unaware of the error. The browser will not necessarily give you any further information unless you ask it to. One way of doing this is with:

use CGI::Carp qw(fatalsToBrowser);

Which will throw errors to the browser. Regards your specific file error, is the permission modes set for ALL users? You may also need to set the directory permissions too. If IIS is running as a guest user, they may not have permission to access the file. This is the same with any webserver and is part of the security measures to ensure protected files cannot be tampered with.