constants.rb

class Klass
  CONST_1 = 1

  class << self
    CONST_2 = 2

    def klass_method
      # ok
      puts CONST_1
      # ok
      puts CONST_2
    end
  end

  def instance_method
    # ok
    puts CONST_1
    # だめ
    puts CONST_2
  rescue => e
    pp e
  end
end

Klass.klass_method
Klass.new.instance_method

# ok
puts Klass::CONST_1
begin
  # だめ
  puts Klass::CONST_2
rescue => e
  pp e
end