Fix GH Issue #19472: read warnings from open($fh,">",\(my $x))

We produce all kinds of warnings if someone opens a scalar reference
that is undef. Prior to this we handled write operations ok, at
least in blead, but read operations would produce a plethora of
warnings. To me this analogous to treating an undef var as hash
and trying to read from it, we autovivify the undef var to be
a hash. So in this case we should just "autovivify" the referenced
scalar to be an empty string.

Eg. before this patch:

./perl -Ilib -wle'open my $fh,"+>", \(my $v); my @x=<$fh>; print 0+@x'
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
0

After it:
./perl -Ilib -wle'open my $fh,"+>", \(my $v); my @x=<$fh>; print 0+@x'
0

1 files changed, 10 insertions, 2 deletions

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index 26d59a580e..1950e95cd0 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -4657,8 +4657,8 @@ L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
 =item Opening a filehandle into an in-memory scalar
 
 You can open filehandles directly to Perl scalars instead of a file or
-other resource external to the program. To do so, provide a reference to
-that scalar as the third argument to C<open>, like so:
+other resource external to the program. To do so, provide an unblessed
+reference to that scalar as the third argument to C<open>, like so:
 
  open(my $memory, ">", \$var)
      or die "Can't open memory file: $!";
@@ -4674,6 +4674,14 @@ The scalars for in-memory files are treated as octet strings: unless
 the file is being opened with truncation the scalar may not contain
 any code points over 0xFF.
 
+Be aware that attempting to open a reference to a readonly scalar will
+cause a warning and the open to fail.
+
+Prior to Perl version 5.36.0, passing in a reference to an undef scalar
+could cause strange warnings. As of Perl version 5.36.0, provided the
+reference is unblessed, the scalar will be "autovivified" to be an empty
+string, even for read mode open operations.
+
 Opening in-memory files I<can> fail for a variety of reasons.  As with
 any other C<open>, check the return value for success.