An even simpler way to do this would be to add all of the numbers up. This also has the performance benefit of avoiding the sort entirely.

For example:-

my $total = 0;
my $expect = 0;

for(int i=0;i<@array;i++) {
    $total += $array[$i];
    $expect += $i;

Correction: $expect += $i + 1;
}

return 1 if ($total == $expect);
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Aaarrrggghhh - sorry, ignore this post. As has been pointed out, it doesn't work.