slightly different formulation, but same idea of linear search. sort the array if it might not be in order.

#!/usr/bin/perl -w
use strict;

my @x = (1001,1002,1003,1004,1006,1007,1009,1010);

print get_next(\@x);  #1005


sub get_next
{
   my $aref = shift;
   
   my $cur;
   foreach (sort{$a<=>$b}@$aref)
   {
      if (!defined $cur){$cur=$_; next;}
      return $cur if (++$cur ne $_);
   }
   return undef;  #search failed - no holes!
}
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