From b6e54476dc978566e3505686d3ce6b18347a4c53 Mon Sep 17 00:00:00 2001
From: bbrtj <bbrtj.pro@gmail.com>
Date: Sun, 14 Dec 2025 20:41:28 +0100
Subject: [PATCH] Fix Perl_av_store off by one error in key comparison

---
 av.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/av.c b/av.c
index 352c1ac034..3a83fec6ff 100644
--- a/av.c
+++ b/av.c
@@ -361,7 +361,7 @@ Perl_av_store(pTHX_ AV *av, SSize_t key, SV *val)
             return NULL;
     }
 
-    if (SvREADONLY(av) && key >= AvFILL(av))
+    if (SvREADONLY(av) && key > AvFILL(av))
         croak_no_modify();
 
     if (!AvREAL(av) && AvREIFY(av))