Thank you for your collective wisdome perl monks. I found This to be quite helpful:

Alternatively, you can make use of the fact the list is sorted (again,
+ untested): 

my @uids = ....;  # List of sorted uids
my $uid = 1001;
for (my $i = 0; $i < @uids; $i++) {
    next if $uids[$i] < $uid;
    last unless $uids[$i] == $uid;
    $uid++;
}
say "First unused uid: $uid";
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My day is going well now!