Prevent misleading undefined value warnings for defelem magic

This could warn within a function was called with a parameter that was
a hash element based on an undef key that didn't exist when that
parameter was referenced.

This also warns more usefully at the point of call.

Author: tonycoz

mg.c

@@ -2616,7 +2616,8 @@ Perl_defelem_target(pTHX_ SV *sv, MAGIC *mg)
     if (LvTARGLEN(sv)) {
         if (mg->mg_obj) {
             SV * const ahv = LvTARG(sv);
-            HE * const he = hv_fetch_ent(MUTABLE_HV(ahv), mg->mg_obj, FALSE, 0);
+            SV * const index_sv = SvOK(mg->mg_obj) ? mg->mg_obj : &PL_sv_no;
+            HE * const he = hv_fetch_ent(MUTABLE_HV(ahv), index_sv, FALSE, 0);
             if (he)
                 targ = HeVAL(he);
         }

t/lib/warnings/mg

@@ -79,3 +79,54 @@ Use of uninitialized value $3 in oct at - line 3.
 use warnings 'uninitialized';
 $ENV{FOO} = undef; # should not warn
 EXPECT
+########
+# NAME Use of uninitialized value $_[0] in defined operator
+# github 22423
+use warnings 'uninitialized';
+sub f { defined $_[0] }
+my $s;
+my %h;
+f($h{$s});
+EXPECT
+Use of uninitialized value $s in hash element at - line 6.
+########
+# NAME Use of uninitialized value $_[0] in defined operator (tied)
+# github 22423
+# should we allow tied hashes to distinguish between undef and ""
+# without warning?  For now test the current behaviour
+use v5.36;
+++$|;
+sub f { defined $_[0] }
+my $s;
+tie my %h, "Foo";
+f($h{$s});
+$h{+undef} = 1;
+$h{""} = 2;
+say $h{+undef};
+f($h{$s});
+
+package Foo;
+
+sub TIEHASH {
+  bless {}, shift;
+}
+sub STORE {
+  my ($self, $index, $val) = @_;
+  $self->{defined $index ? $index : "+undef"} = $val;
+}
+sub FETCH {
+  my ($self, $index) = @_;
+  $self->{defined $index ? $index : "+undef"};
+}
+sub EXISTS {
+  my ($self, $index) = @_;
+  exists $self->{defined $index ? $index : "+undef"};
+}
+
+EXPECT
+Use of uninitialized value $s in hash element at - line 9.
+Use of uninitialized value in hash element at - line 10.
+Use of uninitialized value in hash element at - line 12.
+1
+Use of uninitialized value $s in hash element at - line 13.
+