use strict;
use warnings;

use Socket;

my @IPs = qw{
   10.1.1.5
   10.1.1.6
   172.16.0.0
   192.168.33.17
   192.168.331.54
   192.168.33.74
   172.16.204.255
   0.0.0.0
   255.255.255.255
   10.1.255.255
   };

foreach my $IP ( @IPs )
{
    my $packed = inet_aton( $IP );
    do {
       warn qq{Error: $IP is not a valid IPv4 address\n};
       next;
       } unless defined $packed;
    my $numeric = unpack q{N}, $packed;
    my $isOdd   = substr( $IP, -1 ) % 2;
    $numeric   += ( $isOdd ? 1 : -1 );
    my $newIP   = inet_ntoa( pack( q{N}, $numeric ) );
    printf qq{Old IP: %15s ... New IP: %15s\n}, $IP, $newIP;
}