Usage: wrap.rb any.txt 30

wrap.rb

#!/usr/bin/env ruby
trap('PIPE', 'EXIT')

filnum = ARGV[0]
col = Integer(ARGV[1])

File.read(filnum).lines.each do |line|
  state = 0

  print line.split(//).slice_before {|ch|
    n = ch.ord <= 0xFF ? 1 : 2

    if (state + n) > col
      state = n
      true
    else
      state += n
      false
    end
  }.map(&:join).join("\n")
end