package mybase;
use threads;
use threads::shared;

my $mode : shared;
1;

##</code><code>##

#!/usr/bin/perl
package derived;
use strict;
use Exporter;
use threads;
use threads::shared;
use mybase;
use vars qw(@ISA);
@ISA = ('mybase');

sub foo {
lock($mybase::mode);
$mybase::mode = 1;
}

foo();
1;

##</code><code>##

[root@nf-power2 perltest]# ./derived.pm
lock can only be used on shared values at ./derived.pm line 12.