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