Do you mean that if your array contained:

1001,1002,1003,1004,1006,1007,1009,1010
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You want a function to inspect the array and return 1005?

Exactly.

Unless your array of numbers is huge, I'd use a simple linear search:

sub first { 
    my $aref = shift; 
    $aref->[ $_ ]+1 != $aref->[ $_+1 ] 
        and return $aref->[ $_ ]+1 
        for 0 .. $#$aref-1; 
};;

@a = 1000..1100;;
splice @a, rand(100), 1 for 1 .. 10;;

print @a;;
1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 
+1014 1015 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028
+ ...

print first( \@a );;
1016
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---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Exactly? The array contains 1001, 1002, 1003? Or the array has indexes 1001, 1002, 1003?

For example:

use strict;
use warnings;
use List::MoreUtils qw/firstidx/;

@array = qw/ fred tom jenny jeff claire /;
$array[3] = undef;
my $available_index = firstidx { not defined $_ } @array; 
$array[$available_index] = 'peter';
print "@array\n";
__END__
fred tom jenny peter claire
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Dave

#!/usr/bin/perl

use strict;
use warnings;

my @arr = qw(1000 1003 1004 1012 1013 1019 1020);

my %unused = map {$_ => 1} @arr;
my @av = grep {not $unused{$_}} 1000..1020;
print "@av\n";
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my @uids = ....;   # List of used UIDs
my %uids; @uids{@uids} = ();
my $uid = 1001;
{$uid++, redo if exist $uids{$uid}}
say "First unused uid: $uid";
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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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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!

#!/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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