Huh? How do you call it? This runs fine on my machine:

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);
  $isthere{$_} = "yes" for @list;
  my @vacancies = grep { not $isthere{$_} } sort keys %isthere;
  return \@vacancies;
}

print "@{find_holes([qw(0 1 2 3 4 6 8)])}\n";
[download]

Prints:

d:\tmp\try>perl try.pl
perl try.pl
5 7
d:\tmp\try>
[download]

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