G'day!

Several things to remember.

First of all on Earth, and apparently on Mars too, perchlorate is is a surficial process, formed by UV action of surface minerals. Its probably constantly being broken down and renewed.

Second, Mars does not have a reducing atmosphere, it has a near neutral one, dominated by oxidised (and weakly odixising) CO2. The presence of a soild oxidant, formed by photochemical reactions, is consistent with it.

If Mars had a reducing atmosphere it would have been very early in its history, with the probable presence methane. Under such circumstances perchlorate is unlikely to form because the methane would form photochemical smog through UV action and prevent the UV reaching the surface. If it did form it would probably be consumed through reaction with atmospheric and surface organics.

Lastly, the amount of perchlorate present in proably small, I would be surprised if it is over the 100s of ppm range, and may be less.

Jon