(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).

I'm not sure how to explain "You will have to export things from their respective packages." in different words, so I'll do it by example:

package pack_B;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {third fourth};
...
[download]

This is identical to the respective parts of pack_A, except with the obvious changes for pack_B.

Corion: note that the two packages are sitting in the same module file.
Like I said, it's not straightforward to export from a package which sits lower in a module. Please try to run this module: it doesn't work - it gives errors:

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

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

package pack_B;
use strict;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {third fourth};
sub third {
}
sub fourth {
}
1;
#end package pack_B;
#end of file pack_A.pm
[download]

with the main script:

use pack_B;
# or use pack_A; ?
third();