$x = {
    key0  => {
         a => var1,
         b => var2,
    },
and so on.

##</code><code>##

  hv_store(dusr,"C",1,newSVuv(1),0);
  hv_store(dusr,"B",1,newSVuv(len),0);
  hv_store(dusr,"E",1,newSVuv(now),0);
  hv_store(dusr,"T",1,newSVuv(0),0);

##</code><code>##

  if ((fetch_uv(dusr,"T")) < now) {
    dSP;
    PUSHMARK(SP);
 #  trgt.s is the key for HV * dusr
    XPUSHs(sv_2mortal(newSVpv(trgt.s,4)));
    PUTBACK;
    call_sv(myperlsub,G_VOID);
  }

in the perl routine, this instruction is executed

$x->{trgt}->{T} = 12345;

upon return from the perl call_sv, examination of the contents of the sub hash

  vpp = hv_fetch(hp,trgt.s,1,0);
  value = SvUVX(*vpp);

reveals that the contents appear to contain the initalization value rather than 12345.

HOWEVER, if the hash is iterated

for(itmp = 0; itmp < iend;itmp++) {
  he = hv_iternext(dusr);
  ctmp = hv_iterkey(he,&rtmp);
  ttmp = SvUV(hv_iterval(dusr,he));
  fprintf(stderr,"key %s => %d\n",ctmp,ttmp);
}  

The correct value magically appears

##</code><code>##

SV = IV(0x8380458) at 0x83b70d8
  REFCNT = 1
  FLAGS = (IOK,pIOK,IsUV)
  UV = 0

When the perlsub updates the data it is conveted 
to an NV

SV = PVNV(0x8284298) at 0x83b70d8
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  IV = 0
  NV = 12345
  PV = 0