log

$ ruby test.rb
UTF-8
test_b.rb:8:in `include?': incompatible character encodings: UTF-8 and Windows-31J (Encoding::CompatibilityError)
        from test_b.rb:8:in `block in <main>'
        from test_b.rb:5:in `open'
        from test_b.rb:5:in `<main>'

test.rb

# encoding: cp932
# frozen_string_literal: true
#

File.open 'testdata' do |f|
  while line  = f.gets do
    puts line.encoding
    if line.include?("うう")
      puts line
    end
  end
end

testdata

あ
い
う