Make sure that you have "use Socket" included as showed in demo code. That's the module where you get inet_aton() and inet_ntoa(). (Be careful, this is not the same module as "IO::Socket".)

The code was actually tested and worked. Here is the testing result:

192.168.10.0 has 3 nodes
192.168.1.0 has 2 nodes
10.0.0.0 has 3 nodes
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Update:

Read carric's reply. carric, whenever you copy and paste code, you should remove those '+' signs indicating line breaks. Those are not part of the original code.

$ perl test
syntax error at test line 15, near "+)"
Execution of test aborted due to compilation errors.
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use Socket;
use strict;
use warnings;

my %stat;
my @addr = qw(10.0.0.1
              10.0.0.2
              10.0.0.3
              192.168.1.5
              192.168.1.6
              192.168.10.3
              192.168.10.4
              192.168.10.5);

$stat{inet_ntoa(substr(inet_aton($_), 0, 3) . "\0")} ++ foreach (@addr
++);

print "$_ has $stat{$_} nodes\n" foreach (keys %stat);
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This is what I get. I copy and paste as is as my first test. I am using cygwin. I need to learn more about using Socket because I work a LOT with IP's and subnets. I need to sort IP's right now, so I'm working on that now. Thank you again.

Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY USE_64_BIT_INT USE_LARGE_FILES PE
+RL_IMPLICIT_CONTEXT
  Built under cygwin
  Compiled at Aug 29 2003 12:06:59
  %ENV:
    CYGWIN=""
  @INC:
    /usr/lib/perl5/5.8.0/cygwin-multi-64int
    /usr/lib/perl5/5.8.0
    /usr/lib/perl5/site_perl/5.8.0/cygwin-multi-64int
    /usr/lib/perl5/site_perl/5.8.0
    /usr/lib/perl5/site_perl
    .
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