Checking for undefined in Perl interface code?

PL/Perl tries to handle an "undef" result from a Perl function (which
should convert to a SQL NULL) with code like so:

/* XXX is this the approved way to check for an undef result? */
if (perlret == &PL_sv_undef)
{
retval = (Datum) 0;
fcinfo->isnull = true;
}
else
{
// handle non-null result
}

But I find that it doesn't work, at least not on RH Linux 7.2 with
perl 5.6.0. The if-test fails to notice undef results.

Anyone know what the correct way to do this is?

regards, tom lane