In the first part of Fahrenheit 451, Montag says that kerosene smells like perfume to him. Kerosene is the fuel used by the firemen to burn books, and Clarisse recognizes Montag as a fireman because of this smell.
That Montag compares the smell of kerosene to perfume (something that smells nice) is significant because it shows how much he loves his job at this early stage in the novel. Moreover, the fact that Montag can never fully wash off the smell is also significant. It suggests that firemen are marked out from other people, that they have a special duty that sets them apart from the rest of society. More importantly, it also implies that a man is a fireman for life, not just for a temporary period.
Montag's satisfaction with being a fireman, however, is not as solid as it first appears, as this critical meeting with Clarisse will soon prove.
Montag is a fireman, i.e. "book burner." His job is to extinguish any and all published works which may antagonize the masses and cause unrest. In a nutshell, any dissenting or contrary ideas must be eradicated. As Montag and his neighbor, Clarisse, are conversing, she mentions she does not like the odor of kerosene. It is a defining smell, and Montag describes it as perfume--something that is so much a part of him that he can not ever completely rid himself of the stench of it. Early on in the book, Montag considers it "perfume" as he is intoxicated with his job. He enjoys it and is a dedicated destroyer of ideas. Clarisse opens his eyes, however. He soon learns to despise his work, and begins taking books from the houses he is sent to burn down. He even goes so far as to plant books in a fellow fireman's house on his way out of town.