If you don't need the contents of the file, I would consider installing an auditing tool that monitors the directories of interest. Configure the auditing tool to write out the changes as they occur to some file - and then monitor the output of the auditing tool.
I think this is really where @mmodestino_Splunk is going. And while you can also write a scripted input, that can be hard: how often should your scripted input run and what should it track, etc? Ultimately, your scripted input will be duplicating the functionality of an auditing tool. And good auditing tools already exist for almost every OS.
If you need the contents of the file(s) as well, you could (1) use a monitor input to collect the file contents and place them in one index. And (2) you could also track the output of the auditing tool and place it in a different index in Splunk. So some users might be allowed to read the first index, containing the file contents - while other users are only allowed to search the second index to monitor changes to the files...
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