[perl #7508] Use NULL for nonexistent array elems

This commit fixes bug #7508 and provides the groundwork for fixing
several other bugs.

Elements of @_ are aliased to the arguments, so that \$_[0] within
sub foo will reference the same scalar as \$x if the sub is called
as foo($x).

&PL_sv_undef (the global read-only undef scalar returned by the
‘undef’ operator itself) was being used to represent nonexistent
array elements.  So the pattern would be broken for foo(undef), where
\$_[0] would vivify a new $_[0] element, treating it as having been
nonexistent.

This also causes other problems with constants under ithreads
(#105906) and causes a pending fix for another bug (#118691) to trig-
ger this bug.

This commit changes the internals to use a null pointer to represent a
nonexistent element.

This requires that Storable be changed to account for it.  Also,
IPC::Open3 was relying on the bug.  So this commit patches
both modules.

1 files changed, 2 insertions, 2 deletions

diff --git a/pp.c b/pp.c
index fd40453657..17e0f3bb51 100644
--- a/pp.c
+++ b/pp.c
@@ -4330,7 +4330,7 @@ PP(pp_aslice)
 
 	    svp = av_fetch(av, elem, lval);
 	    if (lval) {
-		if (!svp || *svp == &PL_sv_undef)
+		if (!svp || !*svp)
 		    DIE(aTHX_ PL_no_aelem, elem);
 		if (localizing) {
 		    if (preeminent)
@@ -5016,7 +5016,7 @@ PP(pp_splice)
 	}
 	i = -diff;
 	while (i)
-	    dst[--i] = &PL_sv_undef;
+	    dst[--i] = NULL;
 	
 	if (newlen) {
  	    Copy( tmparyval, AvARRAY(ary) + offset, newlen, SV* );