use strict;
use warnings;

my @ips = qw(
   1.1.1.1    1.1.1.2   1.1.1.127
   1.1.1.128  1.1.1.129
   10.1.2.5   10.1.2.6  10.1.2.10
);

my $mask = 25;

sub pack_ip4   { return pack 'C4', split /\./, $_ for @_ ? $_[0] : $_}
sub unpack_ip4 { return join '.', unpack 'C4', $_ for @_ ? $_[0] : $_}
sub sort_ip4   { map unpack_ip4, sort map pack_ip4, @_ }

my $packed_mask = pack('B32', ('1'x$mask) . ('0'x(32-$mask)));

my %by_net;
foreach my $ip (@ips) {
   my $net = unpack_ip4(pack_ip4($ip) & $packed_mask);
   push @{$by_net{$net}}, $ip;
}

foreach my $net (sort_ip4(keys(%by_net))) {
   print("Network $net/$mask:\n");
   foreach my $ip (sort_ip4(@{$by_net{$net}})) {
      print("  $ip\n");
   }
   print("\n");
}

##</code><code>##

Network 1.1.1.0/25:
  1.1.1.1
  1.1.1.2
  1.1.1.127

Network 1.1.1.128/25:
  1.1.1.128
  1.1.1.129

Network 10.1.2.0/25:
  10.1.2.5
  10.1.2.6
  10.1.2.10