It sounds like you want the big array, before it's sorted, to be all the numbers 1 to n in sequence. Is that right? If so, TIMTOWTDI, but the following will give you an array (@probs) of all the numbers that are not exactly one larger than the one before:

push @probs, grep {pop @array != $array[$#array] + 1} @array;
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§ George Sherston