I'd try it like this: 
#!/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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In reply to Re: Next unused sequential number by Khen1950fx
in thread Next unused sequential number by Anonymous Monk

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