sub parse_ipv4 {
   local *_ = \(@_ ? $_[0] : $_);

   /^([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+)$/
   && $1 < 256
   && $2 < 256
   && $3 < 256
   && $4 < 256
   or return undef;

   return unpack('N', pack('C4', $1, $2, $3, $4));
}