Perlite for "retain Humidity" really? Compared to any other kind of soil perlite is the one of the best option for AERATION purpose, which is the opposite of "retain moisture", although it does retains some, and anything does.
"Why shouldn't I cover it with perlite ?" - You may cover with a piece from the border of this "sponge". Just put on the top, don't push it, the seed must get moisture from all over the shell.