fix heuristics for setsockopt's OPTVAL argument

pp_ssockopt used to treat its last argument (OPTVAL) as a packed string
whenever it has string slot set (SvPOKp), but this would be confused
if the argument is an integer but had cached stringified value.
Now it will treat OPTVAL as a packed string only when it does not
contain any valid public numeric value.

Will fix GH #18642.

pp_sys.c

@@ -2707,7 +2707,7 @@ PP(pp_ssockopt)
     case OP_SSOCKOPT: {
             const char *buf;
             int aint;
-            if (SvPOKp(sv)) {
+            if (SvPOKp(sv) && !SvNIOK(sv)) {
                 STRLEN l;
                 buf = SvPV_const(sv, l);
                 len = l;

t/io/socket.t

@@ -281,6 +281,32 @@ SKIP: {
     ), "0\n", {}, "fresh socket not inherited across exec");
 }
 
+# GH #18642 - test whether setsockopt works with a numeric OPTVAL which also
+# has a cached stringified value
+SKIP: {
+    defined(my $IPPROTO_IP = eval { Socket::IPPROTO_IP() })
+        or skip 'no IPPROTO_IP', 4;
+    defined(my $IP_TTL = eval { Socket::IP_TTL() })
+        or skip 'no IP_TTL', 4;
+
+    my $sock;
+    socket($sock, PF_INET, SOCK_STREAM, $tcp) or BAIL_OUT "socket: $!";
+
+    my $ttl = 7;
+    my $integer_only_ttl = 0 + $ttl;
+    ok(setsockopt($sock, $IPPROTO_IP, $IP_TTL, $integer_only_ttl),
+       'setsockopt with an integer-only OPTVAL');
+    my $set_ttl = getsockopt($sock, $IPPROTO_IP, $IP_TTL);
+    is(unpack('i', $set_ttl // ''), $ttl, 'TTL set to desired value');
+
+    my $also_string_ttl = $ttl;
+    my $string = "$also_string_ttl";
+    ok(setsockopt($sock, $IPPROTO_IP, $IP_TTL, $also_string_ttl),
+       'setsockopt with an integer OPTVAL with stringified value');
+    $set_ttl = getsockopt($sock, $IPPROTO_IP, $IP_TTL);
+    is(unpack('i', $set_ttl // ''), $ttl, 'TTL set to desired value');
+}
+
 done_testing();
 
 my @child_tests;