In one sense, dreams are significant in the plot of In Cold Blood as an indication of how far removed from reality Perry and Dick are.
Perry's dream is to reach Mexico and find a sunken treasure of Spanish gold. This seems to be his ultimate motivation in participating with Dick in the scheme to rob the Clutters: to get the money needed for the trip south of the border. Perry's less specific dream is to make something of his poetry and music and to become a great success as a performer. All of this, of course, is unrealistic and a pure fantasy. Dick's plan for robbery is based on a different dream. He has only the word of another inmate in prison that the Clutters even have a safe in their home, let alone one containing a huge amount of cash. The whole operation is one that only desperate men, whose thinking is disconnected from reality, would attempt to pull off.
The Clutters themselves are in a kind of dream world too. They do not sense their own vulnerability until it is too late. In this, they are no different from most Americans of their time, who lived in a world of ostensible innocence. The overall message of Truman Capote's book may be that a dark undercurrent exists in general in American life unseen by most people. In this case, it is represented by the irrational and sadistic violence unleashed by Perry and Dick. It is significant that toward the end of the story, when the two are on death row, Capote gives an account of their fellow inmate Lowell Lee Andrews, who murdered his parents and sister. Andrews also inhabited a dreamworld; he went to the movies after committing the murders and showed no emotion when questioned by the police. Like Perry and Dick, he is a sociopath cut off from reality, though in the book as a whole, Capote seems to be saying that "normal" people also have their own fantasy world in which they at least partly live.