How to export a package sitting lower in a module?

[HelenCr]

Dear esteemed PerlMonks

I am wrestinlg with the following issue (which may be a trivial one): how do you export functions from a package, which is not the first one in a module file? I've been searching the documentation and the Net, and can't seem to find an answer.
Here is an example:

# Name of this file: pack_A.pm
package pack_A;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {first second};

sub first {
}
sub second {
}
1;
#end package pack_A;

package pack_B;

sub third {
}
sub fourth {
}
1;
#end package pack_B;

In the main script, I would like to use package pack_B, which sits lower in the file: pack_A.pm.
It turns out that it's not that simple. If you add the subroutines 'third' and 'fourth' to @EXPORT,

@EXPORT = qw {first second third fourth};

then in your main script, when you go:

use pack_A;

and call the 'third' sub, it will complain:

undefined subroutine &pack_A::third ...etc

(You can't go:
use pack_B;
since the interpreter looks for the file specified in "use". In other words, it will look for the module pack_B.pm, which does not exist).

Your advice will be appreciated.
Many TIA - Helen