Вот весь код касающийся этого задания

csv.rb

require 'csv'
require 'ostruct'
require 'date'

filename = ARGV.first || 'movies.txt'

abort("Файл не найден") unless File.exist?(filename)

FIELDS = %i[link title year country premiere genre duration rank director cast]

ostruct_movie_arr = CSV.foreach(filename, col_sep: '|', headers: FIELDS )
  .map { |line| OpenStruct.new(line.to_h) }

month_hash = { "1" => 0, "2" => 0, "3" => 0, "4" => 0, "5" => 0, "6" => 0, "7" => 0, "8" => 0, "9" => 0, "10" => 0, "11" => 0, "12" => 0}

month_array = ostruct_movie_arr.reject{ |movie| movie.premiere.length <= 5 }
  .map{|movie| Date.strptime(movie.premiere, '%Y-%m').mon}

uniq_month_array = month_array.uniq

month_array.each do |month|
   month_hash[month] +=1
end

puts month_hash