Closure
This is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside of the context.
they're one of those aspects of Perl -- like object-oriented programming -- that everyone raves about and you can't really see the big deal until you play around with them and then they just click.
References
Closures in Java
Clojure
Clojure is a dynamic programming language that targets the Java Virtual Machine
Clojure Box : An all in one win32 emacs + clojure installer, the quickest way to get started