example.rb

module DSL
  module SomeFeature
    def set_us_up_the_bomb
      raise "Not implemented."
    end
  end
end

module DSL
  class Core
    include DSL::SomeFeature
  end
end

dsl = DSL::Core.new
dsl.instance_eval do
  # Does _not_ raise the exception
  set_us_up_the_bomb
end
# => nil

# Simply returns `nil`, no exception
dsl.set_us_up_the_bomb
# => nil