class Crazy
  def crazytown(&block)
    self.instance_eval(&block)
  end
end

Crazy.new.crazytown do
  ScaryConst = 5
  p ScaryConst
end

p Crazy::ScaryConst
p ScaryConst

## prints
5
rspeq.rb:12: warning: toplevel constant ScaryConst referenced by Crazy::ScaryConst
5
5