sub find_holes { my @list = @{ shift() }; @list = sort { $a <=> $b } @list; my $low = $list[0]; my $high = $list[-1]; my %isthere = map { $_ => 0 } ($low..$high); print "@{[sort keys %isthere]}\n\n"; print "@list\n\n"; $isthere{$_} = "yes" for map {$_+0} @list; my @vacancies = grep { not $isthere{$_} } sort keys %isthere; return \@vacancies; } my @issues = @{ [ '00001', '00002', '00003', '00004', # '00005', '00006', '00007', '00008', '00009', #... ] }; print join("\n", @{ find_holes( \@issues ) });