Compare: to examine two or more things to find similarities and differences. Contrast: to compare two or more things in order to show unlikenesses and differences. Your comparison of characters in A Day in the Life of Ivan Denisovich and Perfume: The Story of a Murderer will do best to focus on the comparison of contrasts. The central characters in Denisovich act according to the constraints of their imprisonments, being driven by external forces, regardless of what they were before they came to be imprisoned. The central character in Perfume is driven by internal forces regardless of external realities.
In addition, the internal forces that direct the central characters in Denisovich and the internal force that compels the major character in Perfume are categorically different. Regardless of their lives before imprisonment, each in Denisovich has internal motivation of surviving the onslaught of maltreatment by captors. The central character in Perfume is internally motivated by greed, selfishness, dominance, cruelty, and a complete imbalance of his comprehension of reality, thus his rather grisly demise. Further, the only internal motivation traits he shares with the jailers in Denisovich is dominance and cruelty: No matter how heinous the war, there is always an accepted reality that the actions of war are necessary.
A derivative theme could relate to one of the following: the balanced or imbalanced view of reality; to the different internal motives; to the different external forces; the struggle to survive (captives, victims); the justice or injustice of various characters' deaths.