# 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";
}