pread_lines.rb

offsets = File.open filename do |file|
  line_offsets = []
  loop do
    line_offsets << file.pos
    file.readline
  rescue EOFError
    break line_offsets
  end
end

File.open filename do |file|
  offsets.each_cons(2).each do |start, finish|
    puts file.pread finish - start, start
  end
end