I'm not sure your 2nd loop does what you intend.

Your code compares $ip_subnet with EVERY entry in the subnets.list file, and gives PASS/FAIL for each.

I think what you want is a PASS if the entry appears anywhere in the list, and fail otherwise.

There's at least 2 ways to do this, one that's good if you're going to have to check a lot of $ip_subnet values against the same stable list, and one for checking just one value, or checking values against a changing list.

# in your initialization...
my %subnets;
open(SUBNET, "<subnets.list") 
    or die "Couldn't open subnets.list for reading, error $!\n";
while(<SUBNET>) {
  chomp;
  $subnets{$_}=1;
}
close(SUBNET);

# for each read $ip_subnet:
if(exists $subnets{$ip_subnet}) {
   print "PASS\n";
}
else
{ 
   print "FAIL\n";
}
[download]

That way, you only read in subnets.list once, and you can reuse it in your code.

If the file is expected to change, or you only need it once, then you'd have to do something like this:

open(SUBNET, "<subnets.list") 
    or die "Couldn't open subnets.list for reading, error $!\n";

my $exists=0;
while(<SUBNET>) {
  chomp;
  if($_ eq $ip_subnet) {
    $exists++;
    last;
  }
}
close(SUBNET);

if($exists) {
    print "PASS\n";
}
else {
    print "FAIL\n";
}
[download]

Hope this helps, and that I understood your problem correctly...
--
Mike