вот прям после первого же примера про stdin в туториале,
The "interest" is represented by an object returned by AnyEvent->io called a "watcher" object - thus named because it "watches" your file handle (or other event sources) for the event you are interested in.
In the example above, we create an I/O watcher by calling the AnyEvent->io method. A lack of further interest in some event is expressed by simply forgetting about its watcher, for example by undef-ing the only variable it is stored in. AnyEvent will automatically clean up the watcher if it is no longer used, much like Perl closes your file handles if you no longer use them anywhere.