When programming with callbacks in perl, you create anonymous functions with
sub { ... }. These functions are especially useful because when they are called
they will preserve their surrounding lexical environment.  Sometimes people
call these anonymous functions that reference variables in their surrounding
lexical scope "closures". Whatever you call them, they are essential for
convenient and efficient asynchronous programming.

For many applications we really like straightforward callback style. The goal
of Callback::Frame is to simplify the management of dynamic environments while
leaving callback style alone.