johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -MSocket -E '
do {
   print qq{Old IP: $_ ... };
   my $isOdd = substr( $_, -1 ) % 2;
   my $IPn   = unpack q{N}, inet_aton( $_ );
   $IPn     += ( $isOdd ? 1 : -1 );
   my $newIP = inet_ntoa( pack( q{N}, $IPn ) );
   say qq{New IP: $newIP};
   } for qw{ 10.1.1.5 10.1.1.6 };'
Old IP: 10.1.1.5 ... New IP: 10.1.1.6
Old IP: 10.1.1.6 ... New IP: 10.1.1.5