How does this differ from Iterator::Util's igrep function?

Excellent question. There is no difference, conceptually. There are some minor differences in implementation, particularly in the interface, as I discuss here. Analogously, my Interface::Array class is the same, conceptually, as Iterator::Util's iarray function. I wrote Iterator::Array, Iterator::Product, and Iterator::Filter partly as demo code, but primarly I wrote them to be transparent (or "white box") building blocks for Implementing Theta Join (select/where) using iterators. No doubt I could have used Iterator::Util instead.