# 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";
}

##</code><code>##

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";
}