require 'csv'

infile = ARGV[0]
slice = ARGV[1] || 100

origin = CSV.open infile
all_lines = origin.readlines

header = all_lines.first
all_lines.delete_at(0)

all_lines.each_slice(slice).each_with_index do |lines, idx|
  CSV.open("splitted_#{idx}.csv", 'w') do |csv|
    csv << header
    lines.each do |line|
      csv << line
    end
  end
end

# $ ruby splitter.rb file.csv 50