You don't need the reverse if you compare reversed values to begin with.

    @b = sort { length $b <=> length $a } @a;
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But then I also wonder if the order of the values _after_ sorting on length might be significant.   You might want to consider something like this:

    @b = sort { length $b <=> length $a 
                           ||
                       $a cmp $b
              } @a;
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