I think the problem here is you haven't really laid out all the possible scenarios. Let's try doing this in English and we'll see where we get:

• If $major_length is 4800, return 18
• Otherwise, If $major_length is 8552:
  • If $val is 4, return 33
  • If $val is 1, return 18
  • Otherwise, ?????
• Otherwise, ?????

You haven't addressed the possibility that (major_length is neither 4800 nor 8852) nor that (major_length is 8852 but val is neither 1 nor 4). Now, it may be that in your application these scenarios are impossible, but the ?: operator does not allow you to simple ignore them. You may find it easier to use an if-else construct instead:

if ($major_length == 4800) {
   $result = 18;
} elsif ($major_length == 8552) {
    if ($val == 4) {
        $result = 33;
    } elsif ($val == 1) {
        $result = 18;
    }
}
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You still have the possibility of those unhandled situations, but now they'll simply result in $result not getting assigned at all.