I need this line

    @tag_prefix_string = @tag_prefix + '.'

So, codes would become as

    if @output_per_tag && !@tag_prefix
      raise Fluent::ConfigError, "tag_prefix must be specified with output_per_tag"
    end
    if @tag_prefix && !@output_per_tag
      raise Fluent::ConfigError, "output_per_tag must be specified with tag_prefix"
    end   
    @tag_prefix_string = @tag_prefix + '.' if @output_per_ta

But, I don't think this is clear. So, let me keep as is.