Perlite will 'float'...but what collects on the top is usually just the stuff that is in the top fraction of an inch of the mix. In other words the very top layer will end up with the lightest stuff on the surface. This could give the appearance of the perlite working its way 'up'...and this usually takes a bit of time to happen, more than one season. I 'fix' it by stirring some 'amendments' into that top fraction of an inch of mix (sprinkle on a little and mix with my fingers).
I've never found significant migration of it in any direction...unless the pot/container is disturbed or flooded...then you have other problems than worrying about what your soil mix is doing.
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