Note: parens around C<grep>'s args are not necessary, so it can be even more concise. ("Elegant", IMHO!)

You should really read

perldoc -f grep
perldoc perlsyn
perldoc perlop

Once you know the elementary charachteristics of each subexpression of the above, combining them should not be too hard.

However grep() will take all the elements from @in and evaluate the !$saw{$_}++ expression for them ($_ is an alias to the actual element). On the first encounter of an item $saw{$_} will be undef, so $saw{$_}++ will return 0 and store one in it. At this point !$saw{$_}++ will return 1 and the element under examination will be passed. A similar analysis for the case in which $saw{$_} already contains a (positive) number (i.e. on second, third, etc. encounter of an item) is left as an exercise to you.