From 2f85065239bee6c5156e2e84547c449ec5584c6f Mon Sep 17 00:00:00 2001
From: Tony Cook <tony@develop-help.com>
Date: Mon, 24 Jul 2023 11:24:17 +1000
Subject: [PATCH] make sure "use VERSION;" sets the flag needed for "used only
 once"

There's two parts to producing the "used only once" warning:

1. In a lexical scope with WARN_ONCE enabled, new GVs are
created with the GVf_MULTI flag off, a second mention of such a
name will set that flag.

2. After compilation, if G_WARN_ONCE is set in PL_dowarn, the entire
package tree is walked to report names which don't have GVf_MULTI set.

In this case G_WARN_ONCE wasn't being set, so the second part didn't
happen.

This flag is global, so using any other module that happened to
enable the WARN_ONCE flag (anything that does C<use warnings;>)
would allow warnings to be produced after compilation.

Fixes #21271
---
 op.c              | 4 +++-
 t/lib/warnings/gv | 7 +++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/op.c b/op.c
index df43c2a02d10..e64fc01aa3e4 100644
--- a/op.c
+++ b/op.c
@@ -7907,8 +7907,10 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
             if (!(PL_hints & HINT_EXPLICIT_STRICT_VARS))
                 PL_hints |= HINT_STRICT_VARS;
 
-            if (shortver >= SHORTVER(5, 35))
+            if (shortver >= SHORTVER(5, 35)) {
                 free_and_set_cop_warnings(&PL_compiling, pWARN_ALL);
+                PL_dowarn |= G_WARN_ONCE;
+            }
         }
         /* otherwise they are off */
         else {
diff --git a/t/lib/warnings/gv b/t/lib/warnings/gv
index 2caf2d36b582..4a8c9aabd82e 100644
--- a/t/lib/warnings/gv
+++ b/t/lib/warnings/gv
@@ -292,3 +292,10 @@ if ($ㄒ) {
 EXPECT
 Name "팣칵ぇ::ʎ" used only once: possible typo at - line 11.
 Use of uninitialized value $팣칵ぇ::ʎ in print at - line 11.
+########
+# https://github.com/Perl/perl5/issues/21271
+use v5.36;
+no strict;
+my $x = $i;
+EXPECT
+Name "main::i" used only once: possible typo at - line 4.