Split num with any digit you want.

split.rb

if ARGV.count < 1
  puts "Usage: ruby split.rb hoge.txt <split_num>"
  exit
end

fname = ARGV[0]
n = ARGV[1]
fn = fname.split(".")
File.open(fname, 'r') do |f|
  File.open(fn[0] + '_r.txt', 'w') do |rf|
    txt = f.read
    a = txt.scan(/.{1,#{n}}/)
    a.each { |s| rf.puts(s)}
  end
end