A.pm
----
# Order doesn't matter.
BEGIN { do 'Constants.pm' }
use B;

##</code><code>##

B.pm
----
BEGIN { do 'Constants.pm' }

##</code><code>##

A.pm
----
# Order doesn't matter.
use Constants;
use B;

##</code><code>##

B.pm
----
use Constants;