Here's some terminology for you. You could look at making Package be a factory. You might also look at removing ISA relationships and using mixins instead. I'd go with the latter given what you just described.

package Package;
use Package::Branch;

...

package Package::Branch;
use Exporter;
*importer = \ &Exporter::importer;
our @EXPORT = qw( get_branch get_trees );

...
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