After some play with Perl (5.8.8 and 5.10.0) I finally have some results:

diff -urN perl-5.8.8/scope.c perl-5.8.8_/scope.c
--- perl-5.8.8/scope.c  2005-09-30 15:56:51.000000000 +0200
+++ perl-5.8.8_/scope.c 2009-02-23 16:23:40.000000000 +0100
@@ -946,14 +946,26 @@
                        SvREFCNT_dec(AvARYLEN(sv));
                        AvARYLEN(sv) = 0;
                    }
+                   av_undef((AV*)sv);
                    break;
                case SVt_PVHV:
                    hv_clear((HV*)sv);
+                   hv_undef((HV*)sv);
                    break;
                case SVt_PVCV:
                    Perl_croak(aTHX_ "panic: leave_scope pad code");
                default:
                    SvOK_off(sv);
+               const U32 padflags
+                 = SvFLAGS(sv) & (SVs_PADBUSY|SVs_PADMY|SVs_PADTMP);
+               switch (SvTYPE(sv)) {   /* Console ourselves with a ne
+w value */
+               case SVt_PVAV:  *(SV**)ptr = (SV*)newAV();      break;
+               case SVt_PVHV:  *(SV**)ptr = (SV*)newHV();      break;
+               default:        *(SV**)ptr = NEWSV(0,0);        break;
+               }
+               SvREFCNT_dec(sv);       /* Cast current value to the w
+inds. */
+               SvFLAGS(*(SV**)ptr) |= padflags; /* preserve pad natur
+e */
+
                    break;
                }
            }
[download]

So yes, I have patched the file scope.c. Because a lot of the (existing) code does not use a explicit undef on a scope exit (of course, does this imply a variable declared with 'my').
At least it helps me to free some malloced space on a scope close (see below).

print "Start: >".$$."<\n";<>;
{
        my @arr;
        $arr[$_]=$_ foreach(1 .. 1000000);
        print "How Many?\n";<>;
#->not needed: undef @arr; 
}
print "Done, how many?\n";<>;
[download]

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