PL/Perl namespace fix

The attached patch appears to fix the problem discussed in the
"plperl gives error after reconnect" thread in pgsql-bugs:

http://archives.postgresql.org/pgsql-bugs/2005-08/msg00133.php

Unfortunately I don't understand *why* it fixes the problem, which
means I'm not sure that it's the correct fix. As Tom Lane and I
discovered, Perl builds with threading don't appear to have a
problem, while Perl builds without threading do. It's not clear
to me why "mksafefunc" works in some Perl installations but
"::mksafefunc" is required in others, given otherwise identical
test conditions. Can anybody explain?

If this is indeed the correct fix then it should probably be
backpatched to 8.0.

--
Michael Fuhr