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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

$VAR1 = {
          '10.0.0.0,30' => {
...
                              'ipn' => 167772172,
                              'maskn' => 4294967292
                            },

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