You said it yourself, you want a script that will monitor the directory so you can't start it from the PHP. Besides you will likely want to run this script and then the icad using a different account than the one the PHP scripts run under.

Create a service (Win32::Daemon or Win32::Daemon::Simple or PDK's PerlSvc) that will either monitor the directory via Win32::ChangeNotify or scan it for changes every now and then and then call the icad and move the files.