sprintf(buff,    "%.19" NVgf, SvNV(arg));
and
sv_setpvf(keysv, "%.19" NVgf, SvNV(arg));

##</code><code>##

sv_setpvf(keysv, "%.19" NVgf, SvNV(arg));
with
sv_setpvf(keysv, "%.19g", SvNV(arg));