Authority::shared allows you to indicate that your module is issued by multiple authorities. the main use case for shared authorities is for team projects. the team would designate a uri to represent the team as a whole, e.g. http://www.perlrdf.org/. releases can then be officially stamped with the authority of the team using:

use authority::shared q;
and users can check they have a module released by the official team using:

rdf::takeovertheworld->authority(q);
which will croak if package rdf::takeovertheworld doesn't have the specified authority.