It seems to "work" for me.

#!/usr/local/bin/perl -w
use strict;

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;
}

my @foo = qw(1 2 5 6 9);
my $vac = find_holes(\@foo);
print "@$vac\n";
__END__

$ ./holes.pl 
3 4 7 8
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