The first numeric sort is 'numerifying' the strings and stripping the leading zeroes which messes up the rest of the routine.

I am not if that is right, at least not in the simple sense of what you say. See:

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 ) });
[download]

Prints:

d:\tmp\try>perl try.pl
perl try.pl
1 2 3 4 5 6 7 8 9

00001 00002 00003 00004 00006 00007 00008 00009

5
[download]

What the sort probably does is to fill the number entry in the glob of the entries of @list. But why does the ".." operator use the number slot, while the hash access code and print use the string slot?

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com