Why does a smell of perfume is spread throughout the room?

The molecules of perfume are in a gaseous state and mixing with the molecules of air in the room. All of them undergo random motion at all times as a result of the internal ("heat") energy that they have absorbed. The molecules often bump into each other, which causes them to mix if they were not mixed previously. Just as air expands to fill all available space, so too do gases in a closed container mix until the mixture is homogenous. This is entropic pressure, a "force" whose existence cannot be proven directly but can be argued statistically. Because the motion of the molecules is entirely random, the probability of finding a given concentration of perfume in a given volume of air approaches an even distribution throughout the room as time goes on. If that explanation was not clear enough, a simple analogy may help. Think of a box that is sparsely filled with two different colors of marbles, with the two colors initially separated. Shake the box. If you keep shaking, the two colors will eventually mix until the separation is entirely destroyed and the distribution is even. This is the same thing that happens to perfume molecules that begin concentrated above the surface of a girl's skin; they gradually disperse until they fill the entire room.