How are sensory perceptions used in "The Wish" to create atmosphere?
"The Wish" concerns a young boy's fantasy in which his carpet is alive with snakes and fire, and he must traverse it to win a prize: a puppy for his birthday. This is a common type of magical thinking, in which a sudden idea turns into an all-encompassing goal. Of the senses, the most important one here is sight, as the boy thinks he can see actual snakes in the carpet. The boy is acutely aware of his need to reach the other side, and the consequences of failing, which become more and more real in his mind. In fact, his fantasy becomes so real that he seems to forget it is fantasy, and becomes utterly terrified that it could be reality, with real consequences:
The mere thought of snakes sent a fine electricity of fear running like pins down the backs of his legs and under the soles of his feet.
He glanced down and saw this deep curling river of black underneath him. Parts of it were stirring now, and uncoiling and beginning to shine with a dreadfully oily glister.
(Dahl, "The Wish," mrsmccschooldays.com)
The boy has convinced himself utterly that his fantasy is true, and so he sees the black areas of the carpet as actual snakes, boiling out of a deep black mess. He can't hear them -- in fact, the text explicitly states that they are "noiseless" -- but he can see them as clearly as he can see the fiery coals in the red parts. This allows his mind to trick his body into feeling the innate panic of danger, the "electricity of fear" which is a common flight-or-fight response. The boy's senses are tricked by his imaginative mind, which seeks a goal known only to himself -- the puppy -- and by his fear of failure, which extends beyond simply stepping wrong and is turned into a mortal fear of actual death.