For source-code-only modules with *nix env. as example

1) Default is using newest awailable module (last installed one).

Storage subsystem:

/usr/lib/perl6/compiler-version/modules/Foo/Bar.pm.0.98.auth_name
/usr/lib/perl6/compiler-version/modules/Foo/Bar.pm.0.98.another_name
/usr/lib/perl6/compiler-version/modules/Foo/Bar/MOP.pm.0.98.auth_name
/usr/lib/perl6/compiler-version/modules/Foo/Bar/MOP.pm.0.99.auth_name

Links to newest modules (or copy of newest modules), updated by package manager, in:

/usr/lib/perl6/compiler-version/lib/Foo/Bar.pm  # ver. 0.98, auth which was last instaled
/usr/lib/perl6/compiler-version/lib/Foo/Bar/MOP.pm # ver. 0.99

So 'use Foo::Bar;' looks in '/usr/lib/perl6/compiler-version/lib' for newest modules.
Means MOP v.0.99 is used by default.

Then 'use Foo::Bar::MOP:ver.a.bc' must be used to get required version. This overrides if newer version
was loaded.

Loader subsystem:

If things must be complicated then middle-ware app can be used for manage set of modules and used as:

perl6 -I manifest.mft  App.pl

or

use Loader::Versions('my_set');