This happens due to diffusion. Due to rapid and random molecular motions and large average inter molecular distance, when two or more nonreacting gases come in contact, they spontaneously mix together to form a homogeneous mixture. When the bottle of the perfume is opened, molecules of the perfume intermix with air spontaneously to form a homogeneous gas mixture. Thus, a person, standing at the other end of the room, can smell the fragrance after a short while.