Try this.

while (<SUBNET> ) {
       chomp; 
       if ($ip_subnet eq $_) { 
             print "PASS\n"; 
       } else { 
             print "FAIL\n"; 
       } 
}
[download]

Update: I realized it would probably be better to also explain what might have been wrong. When you read the subnet.list file into the SUBNET file handle each line gets written into $_ not $subnet. The (<SUBNET>) is just a shortcut for ($_ = <SUBNET>). Also I added the chomp statement to remove any newline characters that would throw off your match. Again using just chomp; is shorthand for chomp $_;.

HTH

-Prime