Thank you gellyfish. I was reading /dev/stdin instead of STDIN because that part is pure cargo from my inspiration at ApacheLogsWithoutIPs. After fixing that and learning from a horde of zombies to call "exit" instead of "next" from inside the child to continue looping in the parent, it seems to function as intended and very efficiently.

#!/usr/bin/perl -w

$SIG{CHLD} = 'IGNORE';

use strict;
use Fcntl qw(:DEFAULT);
use Net::Netmask;
use Net::Whois::IANA;
use Proc::Fork;
use SDBM_File;

my $iana = Net::Whois::IANA->new;

my $bye  = join '|', (
 'strings known to occur',
 'in custname and orgname',
);
my $badhosts = 'some\.bad|other\.worse\d+';

tie (my %seen, 'SDBM_File', '/some/sdbm', O_RDWR|O_CREAT, 0666)
or die "$!";

while (<STDIN>) {
  chomp;
  next if exists $seen{$_};
  $seen{$_} = 1;

  my $ip = $_;

  child {
    my $problem = 0;
    my $host = `host $ip` or die "$!";
    $problem = 1 if $host =~ /$badhosts/i;

    if ($problem or $host =~ /not found/) {
      $iana->whois_query(-ip=>$ip);

      my $inetnum  = $iana->inetnum || '';
      my $custname = $iana->{QUERY}->{custname} || '';
      my $orgname  = $iana->{QUERY}->{orgname}  || '';

      exit unless $inetnum 
      and ($problem or $custname or $orgname);

      if ($problem or 
      $custname =~ /$bye/i or $orgname =~ /$bye/i){
        my $mask = Net::Netmask->new($inetnum);
        `iptables -A INPUT -s $mask -j DROP`;
      }
    }
  exit
  };
}
untie %seen;
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