subnet,prefix,name
10.0.0.0,30,Site ABC
10.0.0.4,30,ISDN dial-up for Ex House
10.0.0.8,30,
10.0.0.12,30,P2P link

##</code><code>##

$VAR1 = {
          '10.0.0.0,30' => {
                             'comment' => 'Site ABC',
                             'ipn' => 167772160,
                             'maskn' => 4294967292
                           },
          '10.0.0.4,30' => {
                             'comment' => 'ISDN dial-up for Ex House',
                             'ipn' => 167772164,
                             'maskn' => 4294967292
                           },
          '10.0.0.8,30' => {
                             'comment' => undef,
                             'ipn' => 167772168,
                             'maskn' => 4294967292
                           },
          '10.0.0.12,30' => {
                              'comment' => 'P2P link',
                              'ipn' => 167772172,
                              'maskn' => 4294967292
                            },

##</code><code>##

foreach my $ip ( keys %subnets ) {
	if (undef $subnets{$ip}{$comment}) {
		$subnets{$ip}{$comment} = 'ALLOCATED';
	}
}