Taint mode in PL/Perl

I've googled and read documentation but I can't figure this out. How
do I configure Postgres so that PL/Perl runs in taint mode? I
understand that plperl runs in "safe mode" but that's not quite the
same thing as taint mode.

I do know that it's not running in taint mode because this script returns 0:

-- are we in taint mode?
create function in_taint() returns varchar as $$
    return ${^TAINT};
$$ language plperl;

select in_taint();

 in_taint
----------
 0
(1 row)

I am using this version of Pg:

PostgreSQL 9.3.9 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu
4.8.4-2ubuntu1~14.04) 4.8.4, 64-bit

I am running it on Linux as follows:

Operating system: Ubuntu Linux 14.04.2
Kernel and CPU: Linux 3.13.0-57-generic on x86_64

I configured PL/Perl in postgresql.conf with this line:

plperl.use_strict 1

Please let me know any other info I can give to help with this issue.
Thanks so much!