On::Event has been split and renamed into MooseX::Event and ONE
MooseX::Event provides an event framework for Moose classes that is inspired by and similar to the one included with Node.js. It provides class helpers to let you declare that you emit named events, methods for you to emit events with and methods to allow users of your class to declare event listeners.
ONE provides a layer on top of AnyEvent that uses MooseX::Event as it's interface. This gives you an arguably nicer, and definitely more consistant interface to write your event based programs.