But why?

testing.rb

class TryIt
  def []=(k,v) return "I'm a return value, if I'm not returned where do I go?" end
end

t = TryIt.new
puts(t['blah'] = "It would be slightly surprising if I were returned, right?")
#=> "It would be slightly surprising if I were returned, right?"

with_a_stack.rb

class TryIt
  def []=(k,v) 
    raise 'look at the stack'
    return "I'm a return value, if I'm not returned where do I go?" 
  end
end

t = TryIt.new
puts(t['blah'] = "It would be slightly surprising if I were returned, right?")

#=>
# testit.rb:3:in `[]=': look at the stack (RuntimeError)
# from testit.rb:9:in `<main>'

# update from sublime
# update from sublime 2