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, 23 insertions, 1 deletions

diff --git a/t/io/open.t b/t/io/open.t
index 7fa492342d..86ca8a8498 100644
--- a/t/io/open.t
+++ b/t/io/open.t
@@ -10,7 +10,7 @@ $|  = 1;
 use warnings;
 use Config;
 
-plan tests => 188;
+plan tests => 193;
 
 sub ok_cloexec {
     SKIP: {
@@ -410,6 +410,28 @@ SKIP: {
         or unlink \*STDOUT;
 }
 
+# [GH Issue #19472] Opening a reference to an undef scalar
+SKIP: {
+    skip_if_miniperl("no dynamic loading on miniperl, so can't load PerlIO::scalar", 5);
+    use strict;
+    my $var;
+    open my $fh, "+>", \$var;
+    is defined($var) ? "defined" : "undef", "defined",
+        '[GH Issue #19472]: open $fh, ">", \$undef_var leaves var defined';
+    is $var, "",
+        '[GH Issue #19472]: open $fh, ">", \$undef_var leaves var empty string';
+    my $warn_count= 0;
+    my $warn_text= "";
+    local $SIG{__WARN__}= sub { $warn_count++; $warn_text .= shift; };
+    my @x= <$fh>;
+    is $warn_count, 0, '[GH Issue #19472]: read after open $fh, ">", ' .
+                       '\$undef_var produces 0 warnings';
+    is $warn_text, "", '[GH Issue #19472]: read after open $fh, ">", ' .
+                       '\$undef_var produces no warning text';
+    is 0+@x, 0,        '[GH Issue #19472]: <$fh> after open $fh, ">", ' .
+                       '\$undef_var returns nothing';
+}
+
 # check that we can call methods on filehandles auto-magically
 # and have IO::File loaded for us
 SKIP: {