use strict;
use warnings;

package B;

our $foo = 3;

package main;

print "foo is >$foo<\n";
__END__
foo is >3<

##</code><code>##

use strict;
use warnings;

package B;
use vars qw($foo);
$foo = 3;

package main;

print "foo is >$foo<\n";
__END__
Global symbol "$foo" requires explicit package name at - line 10.
Execution of - aborted due to compilation errors.

##</code><code>##

use strict;
use warnings;

{
    package B;
    our $foo = 3;
}

package main;
print "foo is >$foo<\n";
__END__
Global symbol "$foo" requires explicit package name at - line 10.
Execution of - aborted due to compilation errors.