use strict;
use warnings;

sub pack_ipv4 {
   my $dotted_ip = @_ ? $_[0] : $_;
   return pack('C4', split(/\./, $dotted_ip));
}

sub unpack_ipv4 {
   my $packed_ip = @_ ? $_[0] : $_;
   return join('.', unpack('C4', $packed_ip));
}

sub get_subnet {
   my ($packed_ip, $subnet_size) = @_;
   my $packed_mask = pack('B32', ('1' x $subnet_size) . ('0' x (32-$subnet_size)));
   return $packed_ip & $packed_mask;
}

my %hashIPs = map { $_ => 1 } (
  (map { "10.0.0.$_" } 0..255),
  (map { "10.0.1.$_" } 0..254),
  (map { "10.0.2.$_" } 0..255),
);

my %count;
for (keys %hashIPs) {
   ++$count{get_subnet(pack_ipv4($_), 24)};
}

for (grep { $count{$_} == 256 } keys %count) {
   print(unpack_ipv4($_), "/24\n");
}

##</code><code>##

my %count;
print("$_/24\n")
   for map unpack_ipv4,
       grep ++$count{$_} == 256,
       map get_subnet($_, 24),
       map pack_ipv4,
       keys %hashIPs;