foreach my $hostname (sort {$hash{$a}->ip_address() cmp $hash{$b}->ip_address() or $a cmp $b} keys %hash) {...}

sub ip_sort {
  my @a = split /\./, $a;
  my @b = split /\./, $b;

  return $a[0] <=> $b[0]
      || $a[1] <=> $b[1]
      || $a[2] <=> $b[2]
      || $a[3] <=> $b[3];
}