It is based on the fact that evaporation causes cooling.This is similar to the cooling of water kept in a mud pot in summer.The mud pot contains very tiny holes through which the water seeps outside.It gets evaporated by the warm air outside.To get evaporated it requires some energy and this energy is the heat energy which is taken from the water itself.So water is getting cooled.This is what happens in our body when the perspiration comes out through the skin holes.
A wet bulb thermometer also works on the principle that evaporation causes cooling...............
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During summer we perspire more to maintain the temperature of our body .Clothes absorb sweat and expose it to the atmosphere and the atmosphere takes the latent heat of vapourisation and the water(sweat) evaporates leaving us cool.
Therefore evaporation causes cooling.
Perspiration is a phenomenon,when we sweat the sweat evaporates taking the latent heat of vaporisation from our body.This phenomenon of taking heat from body surface is called perspiration.Also it happens when we rub alcohol in our skin we feel cool this happens in the same way,alcohol also takes away the latent heat from our body,thus we feel cold.