Just a little simpler and more akin to what he was doing (TMTOWTDI):

#!/usr/bin/perl

use strict;

open(INPUT_FILE, "ipnum");
my @array = <INPUT_FILE>;  #this puts each line of ipnum into an array
close(INPUT_FILE);

for my $line (@array) {
  my @output = `nslookup $line`;
  print "@output";
}
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You might want an appropriately placed chomp() in there somewhere if newlines cause you some problems.