$n = scalar(@aliased);
$a = $_ + rand @aliased - $_ # original code
   = i + rand($n-i) # which becomes....
   = i + choiceof(0, 1, ..., $n-i-1) # perl's rand(10) dumps 0-9 inclusive
   = random integer such that i <= j <= i+$n-i-1
   = random integer such that i <= j <= $n-1
   = random integer such that i <= j <  $n