Orwell lists several types of poor writing in "Politics and the English Language," such as using dying or stale metaphors, using unneeded words and passive voice, and employing pretentious or meaningless words. He then calls this kind of writing a "catalogue of swindles and perversions."
A swindle cheats or defrauds someone of money or possessions: it is a form of stealing that relies on lying. A classic swindle would be persuading a person to invest in a worthless stock or buy a forged painting. Orwell is comparing poor writing to a swindle, saying it is cheating people out of clear thought and substituting a shoddy or worthless product instead. It is a fraud that pretends to have value but really has none.
A perversion diverts something from its original course, corrupting or twisting it. It is usually, in our society, used in a sexual context, meaning to twist what is intended as expression of love to shame, terrify, or hurt someone. Rape can, in this context, be seen as a perversion of intimacy, a crime of violence rather than an act of love. Orwell compares poor writing to a perversion, saying that it corrupts language. Language was invented to express truth, but poor writing twists that purpose by confusing and misleading people.
Swindling is a crime against property, and perversion is a degrading of something good. Orwell says poor writing is both of these, stealing people's ability to think and twisting it into something unwholesome and untrue.