Interesting, but not exactly what im looking for.

Editing your code, I'd like something like this:
(i get the multiple inheritance part, but the functionality is different, top-most subs are not accessed through the base module)

  package Foo;
  use Plugin 'Bar';
  sub shared { print "Shared method" }
  sub foozle { print "Foo's foozle" }

  package Bar;
  our @PLUGIN_SUBS = qw(foozle);
  sub foozle { print "Bar's foozle" }

  # omitting Blip..
  #package Blip;
  #sub foozle { print "Blip's foozle" }

  #=======================
  
  package main;

  $package = 'Foo';
  $package->shared();
  $package->foozle();      # Calls ***Bar's*** foozle
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From what i see theres no other way to have things work that way.

Now this doesnt allow for having two instances of a plugin tree with the same base (at the moment), unfortunately.