The thing to be aware of here is that your case only works OK because numbers are immutable in Ruby. You don’t want to do this with strings, arrays, hashes or pretty much anything else other than numbers, because it would create multiple references to the same object, which is almost certainly not what you want:
a = b = c = d = "test"
b << "x"
=> "testx"
a
=> "testx"
Whereas the parallel form is safe with all types:
a,b,c,d = "test","test","test","test"
=> ["test", "test", "test", "test"]
b << "x"
=> "testx"
a
=> "test"