Inode was my other temptation, but that does little for the actual data inside. Maybe a combination of inode and mtime- but this is only good for a file that stays inside a filesystem. These files may move around, be edited, who knows.

Just to add another perspective, how about getting rid of the filesystem and use some kind of Concurrent_Versions_System? Perhaps a web interface where users can download and upload documents? You could then, while you're at hit, use your "some basic data" and provide your users a search functionality.

Update:
If you decide to go the filesystem route, you may find File::Monitor helpful.