perl 5.6, Data::Dumper, and PL_sv_undef as a hash value

[Will Lowe]

I've got some XS code that needs to return a hash reference.  The hash
keys are integers, and the values will all be "undef".  The relevant
chunk of code looks something like this:

RETVAL = (HV*) sv_2mortal( (SV*) newHV );
/* call some code in an external library */
num_results = find_results( data, &results );
if( num_results ) {
   SV* value;
   for( x = 0; x < num_results; x++ ) {
            value = &PL_sv_undef;
            fprintf(stderr, "XS: returning %u => undef\n", results[x]);
       }
       hv_store_ent(RETVAL, sv_2mortal(newSViv(results[x])), value, 0);
    }
}
OUTPUT:
      RETVAL

... but Data::Dumper is confused. Here's the output of the fprintf()s
above, and what Data::Dumper thinks of the returned hashref, when
results = [3,4,5]:

XS: returning 3 => undef
XS: returning 4 => undef
XS: returning 5 => undef
$VAR1 = {
          '3' => undef,
          '4' => ${\$VAR1->{'3'}},
          '5' => ${\$VAR1->{'3'}}
        };

Here's what Devel::Peek::Dump thinks of the same hashref:

SV = RV(0x8108154) at 0x82a6ce4
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,ROK)
  RV = 0x82aa218
  SV = PVHV(0x82923e0) at 0x82aa218
    REFCNT = 1
    FLAGS = (SHAREKEYS)
    IV = 3
    NV = 0
    ARRAY = 0x82aab38  (0:5, 1:3)
    hash quality = 175.0%
    KEYS = 3
    FILL = 3
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "3" HASH = 0x34
    SV = NULL(0x0) at 0x80f4d18
      REFCNT = 2147482874
      FLAGS = (READONLY)
    Elt "4" HASH = 0x35
    SV = NULL(0x0) at 0x80f4d18
      REFCNT = 2147482874
      FLAGS = (READONLY)
    Elt "5" HASH = 0x36
    SV = NULL(0x0) at 0x80f4d18
      REFCNT = 2147482874
      FLAGS = (READONLY)

Any idea what's up with the "'4' => ${\$VAR1->{'3'}}," stuff?

-- 
        Will