I'm aware that the use of the past perfect tense usually implies a flashback or something similar when writing fiction. Does the entire flashback need to be in past perfect or can it just start out with it?
Clarity is key. All else is negotiable.
The past perfect tense is used to refer to an event that happened in the past and before another event also in the past. For example,
"I had started making brownies when the phone rang."
The past perfect is used to indicate that the start of brownie making is further in the past than the phone ringing.
So let's explore this a little and do a quick story about the brownies and the dresser and see if the past perfect tense is needed throughout:
"I had started making brownies when the phone rang. It was frustrating, trying to get the salesman off the phone. But finally I was able to convey that no, I wasn't interested in buying back issues of National Geographic. But as I hung up the phone, I heard a sound that chilled me to the bone; it was the sound of a dog lapping in a full brownie bowl. It was in that moment that I realized that no brownies were going to get made. I could have gone to the store and bought some there and I had intended to do it, but then I remembered that only home-baked items were permitted."
In this case, we've set the mood with the first sentence and the timeline. As we progress through the storyline, we shift through a variety of verb tenses, even having present tense in there. I could have written in a use of future tense as well (so the retelling of the story, I could have indicated future events and/or actions).
So the answer to the question is - no, the flashback need not be all in past perfect, and in fact that would hamstring your ability to tell the story of the flashback.
The advice I'd like to leave here is that, as in the use of any sort of language and not just writing, the most important thing is to bring clarity to the reader - to make sure that what it is you want the reader to know is conveyed in the way that will make that possible.