Slices.

For example, let @f and @g contain permutations of the numbers 0..$#f. Then the product (composition) of these permutations can be obtained in one step as @f[@g].

It is also easy to obtain the inverse permutation (swap indices and values):
@h[@f] = 0..$#f
(if there is no need to keep the source permutation, then @f itself can be used instead of @h).

Assignment operators as lvalues.

I've always wanted to have in a programming language the ability to do something like this:
($x += 2) *= 3