This ends up with horrible infinite looping, so is a bit broken ;)

find_references.rb

module ObjectSpace
  class << self
    def find_references(target)
      each_object.select {|obj| references?(obj, target) }
    end

    def references?(object, target)
      contains = case object
      when Hash
        object.any? {|k, v| references?(k, target) || references?(v, target) }

      when Enumerable
        object.any? {|e| references?(e, target) }

      when Proc
        if object.source_location.nil?
          false # C code
        else
          locals = object.binding.eval("local_variables")
          locals.any? {|l| object.binding.eval(l.to_s).equal? target }
        end
      end

      contains ||
          object.instance_variables.any? {|v| object.instance_variable_get(v).equal? target } ||
          ((object.is_a? Class) && object.class_variables.any? {|v| object.class_variable_get(v).equal? target })
     end
  end
end

x = Object.new

ObjectSpace.find_references x