gistfile1.rb

class Context
end

Context.new.instance_eval <<-END
  class String
    def foo() 'foo' end
  end
  puts ''.foo
END

#=> undefined method `foo' for "":String (NoMethodError)

-----
class Context
end

Context.new.instance_eval do
  class String
    def foo() 'foo' end
  end
  puts ''.foo
end

#=> foo