use c1;
print(&C1, C2(), C3, "\n");   # blahfoobar  BINGO!

do "c2.pm";
print(&C4, C5(), C6, "\n");   # blahfooC6   PARTIAL FAIL!

BEGIN { do "c3.pm"; }
print(&C7, C8(), C9, "\n");   # blahfoobar  BINGO!

c1.pm
=====
package c1;
require Exporter;
use base qw(Exporter);

@EXPORT = qw(C1 C2 C3);
use constant C1 => 'blah';
use constant C2 => 'foo';
use constant C3 => 'bar';
1;

c2.pm
=====
use constant C4 => 'blah';
use constant C5 => 'foo';
use constant C6 => 'bar';

c3.pm
=====
use constant C7 => 'blah';
use constant C8 => 'foo';
use constant C9 => 'bar';