How did you learn of this, and what have you used this for previously?

I read the standard docs on pack and unpack and did experiments to learn the many aspects that aren't clearly explained. Acme::ESP uses parts of this. I've also used it several times to construct arguments to be passed through Win32::API. I also read the standard sysread docs and have used the 4th argument to efficiently append more to a buffer holding a scrolling window of data from a file.