The smell of perfume spreads out by a process known as diffusion. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. In this case, perfume molecules in the air move and spread out to fill the entire space. This is a passive process that does not require energy and is described by Fick's laws. The rate of diffusion can be influenced by temperature and the size of the particles.

Final Answer: $diffusion$