use strict;
use warnings;
package MyMod;
use Exporter ();
use MyMod::db;

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

use strict;
use warnings;
package MyMod::db;
# I also tried use MyMod qw(MYMOD_DEFAULT) when constant was
# in EXPORT_OK and not EXPORT.
use base qw(MyMod);
sub fred {
        print "hello" if (MYMOD_DEFAULT);
}
1;
[download]

Bareword "MYMOD_DEFAULT" not allowed while "strict subs" in use at MyMod/db.pm line 6. Compilation failed in require at MyMod.pm line 5. BEGIN failed--compilation aborted at MyMod.pm line 5. My original (before simplification) was using EXPORT_TAGS.

I guess we're all reading these a little too fast. And I think I confused you with my comment about OO and use base. You don't need that for what you're doing, though you do need it for Exporter which we overlooked before.

Here's an annotated version of something that should work. I've reordered/simplified lines for clarity. For example, you don't really need the BEGIN block and assuming you're using Perl 5.6 or later, you can replace all the use vars with our.

# 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;
[download]

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

use MyMod; # imports MYMOD_DEFAULT automatically

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

1;
[download]

# fred.pl
use strict;
use warnings;

use MyMod::db;

MyMod::db::fred(); # fully qualified call
[download]

Output of fred.pl:

hello
[download]

-xdg

Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

Thanks, that does in deed work but it has strayed a somewhat from what I was trying to do which is probably because I tried to simplify too much. I am actually trying to subclass Perl DBI so I need:

DBIx::MyMod
DBIx::MyMod::db
DBIx::MyMod::st
[download]

The user of DBIx::MyMod does

use DBIx::MyMod qw(MYMOD_CONSTANT);
# connect in the root
my $h = DBIx::MyMod->connect(x,y, {XXX => MYMOD_CONSTANT});
[download]

I override connect in DBIx::MyMod and other DBI methods in DBIx::MyMod::db and DBIx::MyMod::st. I want to define and export the constants in DBIx::MyMod but still use them in DBIx::MyMod::db. I believe this would be equivalent to your fred.pl and the previous MyMod example doing:

fred.pl
use MyMod; # assuming all constants in EXPORT

MyMod::db->fred();
[download]

and

use strict;
use warnings;
package MyMod;
use MyMod::db;

use constant MYMOD_DEFAULT => 1;
use base qw (Exporter);
our @EXPORT = qw(MYMOD_DEFAULT);

1;
[download]

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

Do you know if it is possible to make that work?