nopescope.rb

class Foo
  def self.const_missing const
    const
  end

  def method_missing *args
    p FOO
    args
  end
end

p Foo.new.instance_exec { foo 1, 2, 3 }
p Foo.new.instance_exec { FOO }

# :FOO
# [:foo, 1, 2, 3]
# foo.rb:13:in `block in <main>': uninitialized constant FOO (NameError)
#         from foo.rb:13:in `instance_exec'
#         from foo.rb:13:in `<main>'