The problem is with your argument to the find_holes sub. You're returning an array reference, so why not accept an array reference? Try this:

sub find_holes {
  my $ref = shift;
  @list = @$ref;
  @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;
}
[download]

This worked as far as I could tell. Hope it helps.

~CS

Update: The others are quite right... this does work they way you had it before. *sigh*