I have thought again about this problem, and it seems to me that you could have avoided this problem by using an array instead of a hash in your sub - this will get you the correct (numeric) comparison and save you one sort:

sub find_holes {
  my @list = @{ shift() };
  @list = sort { $a <=> $b } @list;
  my $low = $list[0];
  my $high = $list[-1];
  my @notfound;
  @notfound[$low..$high] = ($low..$high);
  $notfound[$_] = 0 for @list;
  my @vacancies = grep {$_} @notfound;
  return \@vacancies;
}
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Since the value of the array index is not accessible in a simple for loop, I set the values of the array equal to the index and inverted the sense of the final grep, but otherwise everything is as it was. Note that this code will fail (just like the original one) if 0 is contained in the interval $low .. $high. But this can easily be fixed by setting the values we found to undef rather than 0 and checking for defined values in the final grep...

pike