MyMod.pm
========
use strict;
use warnings;
package MyMod;
use Exporter ();

use vars qw (@EXPORT @EXPORT_OK);
use constant MYMOD_DEFAULT => 1;
BEGIN {
    @EXPORT = ();
    @EXPORT_OK = (MYMOD_DEFAULT);
};
use MyMod::db;
1;

MyMod/db.pm
===========

use strict;
use warnings;
package MyMod::db;
sub fred {
        print "hello" if (MYMOD_DEFAULT);
}
1;