# MyMod.pm
use strict;
use warnings;
package MyMod;

use constant MYMOD_DEFAULT => 1; 

use base qw( Exporter );           # You need to subclass Exporter
our @EXPORT = qw( MYMOD_DEFAULT ); # No extra parens needed

1;

##</code><code>##

# MyMod/db.pm
use strict;
use warnings;
package MyMod::db;

use MyMod; # imports MYMOD_DEFAULT automatically

sub fred {
        print "hello" if (MYMOD_DEFAULT);
}

1;

##</code><code>##

# fred.pl
use strict;
use warnings;

use MyMod::db;

MyMod::db::fred(); # fully qualified call

##</code><code>##

hello