You don't tell us which operating system(s) you are using. If you're using a recent operating system like Windows NT, 2000 or XP, then there is Win32::ChangeNotify, which will allow you to conveniently monitor a directory or a whole tree. For other operating systems, like IRIX or (I believe) Linux, there is SGI::FAM, which also gives you notifications on file system changes.

I guess that NFS doesn't give you any notifications, so if you mount your drives via NFS, I guess you're out of luck.

So, if none of the options apply to your situation, you'll have to come forth with more details.