I almost got it to work but no dice. It matches everything. Must be the if line. What does the regex m{($RE{net}{IPv4})}g actually do here? I know what the line means from the documentation for that module, but what is the exact purpose in this script?

use strict;
use warnings;
use Regexp::Common qw( net );

open ( IPS, '<', "TEST_IPS.txt" ) || die "can't open IPS!";
open ( RULES, '<', "test_rules.txt" ) || die "can't open file!";

my %interesting_ips;

while( <IPS> ) {
  chomp;
  $interesting_ips{ $_ } = 1;
}

while( my $fw_line = <RULES> ) {
  if ($fw_line =~ m{($RE{net}{IPv4})}g ) {
    print ( $fw_line);
  }
}
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