Authen::Simple::PAM doesn't DWIM

[Simon Wistow]

I have a script like

-- >8 --

#!/usr/local/bin/perl -w

use strict;
use Authen::Simple::PAM;

my $service  = 'login';
my $user     = shift || "";
my $pass     = shift || "";
my $pam = Authen::Simple::PAM->new(
        service => $service
);

if ( $pam->authenticate( $user, $pass ) ) {
    print "Success!\n";
} else {
    print "Fail!\n";
}

-- >8 --

Which, if I run it from the command line succeeds all the time as long 
as $user is the same user I'm logged in as, no matter what the password 
given is.

However, if I give another username then no matter whether the password 
is correct or not, it always fails.

Am I missing something really obvious? Is PAM not supposed to work this 
way? Is it something screwy with my FreeBSD jail set up? 

Confused of North London