class A
  def foo ; p "a" end
  def bar ; foo end
end
class B
  def foo ; p "b" end
end
B.new.instance_exec &A.new.method(:foo).to_proc # prints "a", not "b"
B.new.instance_exec &A.new.method(:foo)         # idem

$p = proc do ; foo end
B.new.instance_exec &$p                         # prints "b" (ok)