don't warn on 'undef $^W'

mg.c

@@ -3152,9 +3152,8 @@ Perl_magic_set(pTHX_ SV *sv, MAGIC *mg)
     case '\027':	/* ^W & $^WARNING_BITS */
         if (*(mg->mg_ptr+1) == '\0') {
             if ( ! (PL_dowarn & G_WARN_ALL_MASK)) {
-                i = SvIV(sv);
                 PL_dowarn = (PL_dowarn & ~G_WARN_ON)
-                                | (i ? G_WARN_ON : G_WARN_OFF) ;
+                                | (SvTRUE_NN(sv) ? G_WARN_ON : G_WARN_OFF) ;
             }
         }
         else if (strEQ(mg->mg_ptr+1, "ARNING_BITS")) {

t/lib/warnings/mg

@@ -169,3 +169,8 @@ EXPECT
 Use of uninitialized value $s in hash element at - line 13.
 Use of uninitialized value $_[0] in defined operator at - line 10.
 defined
+########
+# NAME undefined $^W shouldn't warn
+use warnings;
+undef $^W
+EXPECT

t/op/reset.t

@@ -197,7 +197,7 @@ SKIP:
         my $warn = '';
         local $SIG{__WARN__} = sub { $warn .= "@_\n" };
         reset "\cW";
-        like($warn, qr/uninitialized/, "magic tries to SvIV() the new value");
+        is($warn, "", 'no warnings resetting $^W');
         $warn = '';
         is($^W, 0, q"check $^W has been zeroed");
         is($warn, '', "should be no more warnings");