use Foo::Bar;
use Devel::InnerPackage qw(list_packages);
my @inner_packages = list_packages('Foo::Bar');
package Foo::Bar;
sub foo {}
package Foo::Bar::Quux;
sub quux {}
package Foo::Bar::Quirka;
sub quirka {}
1;
then
list_packages('Foo::Bar');
will return
Foo::Bar::Quux
Foo::Bar::Quirka
Distributed under the same terms as Perl itself.