The soil my friend got was pre-mixed with the perlite. Sterilized so no insects (or so they claim-but I'll believe it)
You can mix your own perlite soil mix by preparing a large container or empty cooler, as long as you're measuring exactly as much soil as you are perlite. You can also mix 60/40 soil/perlite. How much total depends on the size of your pots.
You can even premix a bunch and keep it in a bag or some other container and it will be ready for use whenever you transplant or plant new seeds
Something to keep in mind when mixing the perlite is that it creates alot of dustin the air from the little balls mixing around, so make sure to do the mixing in a well ventilated area near a window or outdoors.
Perlite
Perlite creates space within the medium to allow for the roots to stretch easier and farther around the medium. It helps the soil drain well as well so you may need to water slightly more often than u were before using the perlite