Subnets aren't really that complicated if you convert them to integers. I haven't really tested this code a lot but it should work for any contiguous netmask and valid subnet/netmask combination. You may want to add some error-checking for that.

use Socket;

sub next_subnet {
  my ($network, $subnet) = @_;

  my $inet = unpack('N', inet_aton($network));
  my $mask = unpack('N', inet_aton($netmask));
  my $ones = unpack('N', inet_aton('255.255.255.255'));

  my $size = ($ones ^ $mask) + 1;
  my $next = $inet + $size;

  return inet_ntoa(pack('N', $next));
}
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You may find this handy too:

sub prefix_to_mask {
  my ($bits) = @_;

  return inet_ntoa(pack('N', (2**32) - (2**(32-$bits))));
}
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Converting from netmask to prefix length is left as an exercise :-)

Edit: I almost forgot, you have to use Socket; for inet_aton/inet_ntoa.