Actually, you still have two arrays in memory. print for (sort @ary); creates a new array to hold the results of the sort, and can't throw out the value of @ary (until after the script ends), because the variable @ary is still in scope. @ary=sort @ary doesn't have this problem, in theory, but up to somewhen in the 5.8.x series (5.8.2, IIRC), it created a temporary array anyway.

In any case, yes, native sort is probably faster then perl sort for this.