> I wonder why.

IMHO legacy code would break if you die here.

But a warning would be great.

I agree on a warning, it would be a practical thing, after all. And after that, a real fix with 'die' a couple versions later should help the legacy code.

Well anyway, I found the code suggested by ikegami, so would you guys review the patch please?

--- pp.c.0    2012-01-06 01:07:34.511988700 +0100
+++ pp.c    2012-01-06 19:04:06.311428300 +0100
@@ -515,9 +515,11 @@
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }
-    else if (SvPADTMP(sv) && !IS_PADGV(sv))
+    else if (SvPADTMP(sv) && !IS_PADGV(sv)) {
+        Perl_ck_warner(aTHX_ packWARN(WARN_MISC),
+         "Implicit copy of a read-only scalar due to aliasing");
         sv = newSVsv(sv);
-    else {
+    } else {
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }
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Negative. You're are making it consistent by breaking the working case. See Re^4: ref to read-only alias ... why? (not consistent).