Oh my eyes.. :) I suggest replacing main with

    def file_with_split_sentences(old)
      contents = File.read(old).split_keep_after(/[\.!\?] *[A-Z\n]/, 2)
      begin newname = "#{old}-sdiff-#{random_chars 5}"; end while File.exists?(newname)
      File.open(newname, "w") { |file| file.write contents.join("\n")}
      newname
    end

    fnames = ARGV[-2..-1].collect{|f| file_with_split_sentences f.chomp}
    system "diff #{ARGV[0...-2].join " "} #{fnames.join " "}"
    "rm #{fnames.join " "}"