Cannot create perlplu function

Running PostgreSQL 9.1.8 on Xubuntu 12.04, installed from the repos.

From the shell, I have executed:

createlang plperl db_name;
createlang plperlu db_name;

As the superuser running psql, I have executed:

GRANT ALL ON LANGUAGE plperl TO account_name;
GRANT ALL ON LANGUAGE plperlu TO account_name;

The pg_language table reveals:

select lanname,lanpltrusted from pg_language where lanname like 'plperl%';

"plperl";t
"plperlu";t

When I create the following function:

CREATE OR REPLACE FUNCTION get_hostname()
  RETURNS text AS
$BODY$
  use Sys::Hostname;
  return hostname;
$BODY$
  LANGUAGE plperlu IMMUTABLE
  COST 1;
ALTER FUNCTION get_hostname()
  OWNER TO account_name;

I receive the following error:

ERROR:  Unable to load Sys/Hostname.pm into plperl at line 2.
BEGIN failed--compilation aborted at line 2.
CONTEXT:  compilation of PL/Perl function "get_hostname"

Yet the following works:

CREATE OR REPLACE FUNCTION get_hostname()
  RETURNS text AS
$BODY$
  return '127.0.0.1';
$BODY$
  LANGUAGE plperlu IMMUTABLE
  COST 1;
ALTER FUNCTION get_hostname()
  OWNER TO account_name;

The following Perl script works as expected from the shell:

use Sys::Hostname;
print hostname;

I tried to run the function as an anonymous block:

DO $$
  use Sys::Hostname;
  print hostname;
$$ LANGUAGE plperlu;

This returned the same error as before, with this additional information:

ERROR: Unable to load Sys/Hostname.pm into plperl at line 3.
BEGIN failed--compilation aborted at line 3.
SQL state: 42601
Context: PL/Perl anonymous code block

From the documentation, error 42601 is a syntax error.

I see no syntax error.

Furthermore, the function loaded fine with the superuser account when I imported the database:

psql -d db_name -U account_name -W -f db-dump.sql > import.log 2> error.log

I tried to simplify the code to no avail:

CREATE OR REPLACE FUNCTION get_hostname() RETURNS text AS
$$
  use Sys::Hostname;
  return hostname;
$$
LANGUAGE plperlu;

Same error as before.

What I don't understand is why, according to the error message, PostgreSQL is trying to load the code into plperl instead of plperlu.

Any ideas?

Update #1

Note that the postgres user is a Superuser:

                             List of roles
 Role name |                   Attributes                   | Member of 
-----------+------------------------------------------------+-----------
 postgres  | Superuser, Create role, Create DB, Replication | {}