scope.rb

module Scope
  module ScopeMethods
    def helpers(&block)
      instance_eval(&block)
    end
  end

  def scope(path, &block)
    (@path_parts ||= []) << path

    context = eval('self', block.binding).dup
    context.extend ScopeMethods 
    context.instance_eval(&block)

    @path_parts.pop
  end
end

include Scope

scope "something" do
  helpers do
    def foo
      "bar"
    end
  end

  puts foo # => should work
end

puts foo # => should error

You need to refactor the scope() method to the following:

  def scope(path, &block)
      (@path_parts ||= []) << path

      context = eval('self', block.binding)
      dup_context = context.dup 
      dup_context.extend ScopeMethods 
      dup_context.instance_eval(&block)

       # update modified state
       context.instance_variables.each do |v|
          context.instance_variable_set(v, dup_context.instance_variable_get(v))
      end

     @path_parts.pop
end 

Let me know if your tests pass with that