I need to write a subnet mask calculator with perl. The extension Net::IP is really powerful.

Well, you could use that module. First thing to do is read its documentation, and off you go. Come back with some code to discuss.

Alternatively I just happen to have a calculator in my toolbox for calculating network, netmask and broadcast out of an IPv4 address in CIDR notation which could be worth being studied by you:

#!/usr/bin/perl
# file cidr
$m=pack B32,pop=~'/'x$';
printf"$` network %vd broadcast %vd netmask %vd\n",($z=eval$`)&$m,$z|~
+$m,$m;
__END__
Example:
cidr 209.197.123.153/27
209.197.123.153 network 209.197.123.128 broadcast 209.197.123.159 netm
+ask 255.255.255.224
[download]

  use Net::Netmask;

  $range = undef;
  $range = "$ip/$maskip";
  print "-- RANGE: \"$range\"\n";
  $msess = undef;
  $msess = new Net::Netmask($range); 

  $table = undef;
  $msess->storeNetblock($table);
  $wireip = undef;
  $wireip = $msess->base();
  $bcastip = undef;
  $bcastip = $msess->broadcast();
 
  if($debug == 1)
    {
    print "WIREIP: \"$wireip\"\n";
    print "MASKIP: \"$maskip\"\n";
    }

   # Dump all ips in range
   $oip = undef; 
    foreach $oip ($msess->enumerate(0))
      { 
      print "OIP: \"$oip\"\n"; 

      }
[download]