[Kolyaj]

((0)['constructor']['constructor'](
      ( 
        (0)['constructor']['constructor']
        ('return unescape')
      )()
   
    ('al%65rt%28%22Hello%20World%21%22%29')
  )
)();

(0)['constructor'] == Number
Number['constructor'] == Function

(Function(
      ( 
        Function
        ('return unescape')
      )()
   
    ('al%65rt%28%22Hello%20World%21%22%29')
  )
)();

Если растянуть

(Function((Function('return unescape'))()('al%65rt%28%22Hello%20World%21%22%29')))();

Function('return unescape') == function() { return unescape; }
(Function('return unescape'))() == (function() { return unescape })() == unescape

(Function(unescape('al%65rt%28%22Hello%20World%21%22%29')))();

(Function('alert("Hello World!")'))();

(function() { alert("Hello World!") })();