package A;
    use strict;
    use vars qw/@EXPORT @ISA $problem/;

    use Exporter;
    @ISA = qw/Exporter/;
    @EXPORT = qw/$problem/;

    1;

##</code><code>##

    package B;
    use strict;
    use vars qw/@ISA @EXPORT/;

    use A;
    use Exporter;
    @ISA = qw/Exporter/;
    @EXPORT = qw/foo/;

    sub foo { print "problem is $problem\n" }

    1;

##</code><code>##

    use A;
    use B;

    $problem = "bar";
    foo();