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";
}