CSV を文字コード変換しつつロード ref: http://qiita.com/items/8559576b71642b79df67

file0.rb

CSV.foreach(filename, encoding: "Shift_JIS:UTF-8") do |row|
  # …
end

file1.rb

open(filename, "rb:Shift_JIS:UTF-8", undef: :replace) do |f|
  # …
end

file2.rb

open(filename, "rb:Shift_JIS:UTF-8", undef: :replace) do |f|
  CSV.new(f).each do |row|
    # ...
  end
end