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   parent 1: "aaaaaaaaaaaaaa"  (these are only copies -- the parents a
+re
   parent 2: "AAAAAAAAAAAAAA"      not modified in this operation)
...
      and then swap sections..
   result 1: "aaaaaaAAAAAAaa"
   result 2: "AAAAAAaaaaaaAA"

use strict;
use List::Util qw/shuffle sum/;
...
sub rand_string {
    join "" => map { rand() > 0.5 ? 0 : 1 } 1 .. $str_length;
}