You can also tie() a hash to a persistent data store such as a Berkely-DB file.   You use hash-syntax in your program, but the data store is persistent.   BDB is a fast-and-efficient VSAM implementation.

Various operating system interfaces exist to notify you when a file has changed.   But as I am reading this, I find myself thinking ... “isn’t this the perfect spot to use a pipe?”   (Yes, even Win32 has them.)

Ahh, yes.   TSO...   I remember it, but not “well.”