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