"Perked up" is an idiomatic phrase commonly used to describe one's state of heightened alertness or energy. The opposite of perked up can be described in several ways depending on the context. If we use "perked up" to refer to someone's mood or spirits, then the antonyms could include disheartened, discouraged, or demoralized. If we are using "perked up" to describe a physical stance, then terms such as slouched, drooped, or collapsed could work as antonyms. To contrast perked up regarding attentiveness, the phrases missed or ignored when used in different language and context. However, in essence, the antonyms of "perked up" are dependent on what the term is describing.