When you ate all the bread and then remember the good times when you still had the bread
1) Is the word 'still' redundant in this sentence?
2) Should I use the past perfect tense in the second part? (e.g. when I had had the bread)
"Still" is not redundant, although removing this word will still convey a very similar meaning. The use of "still" is an alternative way of invoking the past perfect tense without using the very awkward form "when I had had the bread"-- the double "had" is grammatically correct, but native English speakers find this construction to be weird and confusing, even humorous. As a general rule, we hate repetition!
To clarify, the use of "still" is not ACTUALLY the past perfect tense, but it invokes the same idea. Saying "still" makes your sentence talk about an extended period of time in which the bread existed, and putting it in the past tense with "had" means that "still" is no longer the case right now-- meaning that there was a point in the past where "still" ceased to be. This is the same meaning as using the past perfect, without actually being past perfect!
The sentence is still grammatical if you use neither "still" nor the past perfect tense: "When you ate all the bread and then remember the good times when you the bread" is a valid sentence, but in this form, it no longer draws attention to that moment when you lost the bread. The original sentence emphasizes that moment, but without "still" or the past perfect, we're just talking about the past, not the moment that the past changed.