dog.rb

class Dog
  def name=(x)
    trace_ivar_set(:@name, x)
  end

  def trace_ivar_set(ivar, x)
    puts "#{ivar} is set to #{x.inspect} for #{self.inspect} at #{caller[1]} called from #{caller[2]}"
    instance_variable_set(ivar, x)
  end

  def name_taro
    self.name = 'Taro'
  end
end

a = Dog.new
a.name_taro
p a

$ ruby dog.rb
@name is set to "Taro" for #<Dog:0x007fbf3c2861c8> at dog.rb:12:in `name_taro' called from dog.rb:17:in `<main>'
#<Dog:0x007fbf3c2861c8 @name="Taro">