Heay I wrote a program that deals with detecting changes in a file system.

Check out my Program Archiver. Specifically check out the make_comp subroutine.

The basics of how it works is that it gets the current files in a subdirectory.

Then  it reads a file that has the previous directory contents. 

Then it generates a comparison so it can know if a files is new, changed, unchanged, or deleted.

Hope this helps.