### External.pm
package External;

use strict;

use base qw(Exporter);
use vars qw(@EXPORT);
@EXPORT = qw(external);

sub external {
    print "I'm in the external package!\n";
}

1;

### Foo.pm
package Foo;

use strict;

use External;

use base qw(Exporter);
use vars qw(@EXPORT);
push @EXPORT, @External::EXPORT;

sub from_base {
    print "In base...\n";
    external();
    print "Done in base...\n";
}

1;

### Bar.pm
package Bar;

use strict;

use Foo;
use vars qw(@ISA);
@ISA = qw(Foo);

#use base qw(Foo);

my $x = bless { }, 'Bar';

$x->from_base();
external();

1;

### base-tester
#!/usr/bin/perl -w

use strict;

use Bar;

print "Done\n";