Which method of internal processing looks or feels best? Were the internal methods used have side effects, or no?

no_side_effects.rb

def Cherry
  def initialize(cherry)
    @cherry = cherry
  end
  
  def process
    @cherry = remove_pit(@cherry)
    @cherry = twist_stem(@cherry)
  end

private
  def remove_pit(cherry)
    cherry.delete('pit')
  end
  
  def twist_stem(cherry)
    cherry['stem'].twist
  end
end

side_effects.rb

def Cherry
  def initialize(cherry)
    @cherry = cherry
  end
  
  def process
    remove_pit
    twist_stem
  end

private
  def remove_pit
    @cherry.delete('pit')
  end
  
  def twist_stem
    @cherry['stem'] = @cherry['stem'].twist
  end
end

Going with internal instance methods that have side effects.

The whole point in an instance having internal state is to be able to use it. It makes sense that internal methods are going to manipulate that state. If we're passing around methods everywhere, it might as well be a private class method, which is silly.