It would seem to me that the plugin isn't the class, but the object, or the instantiation of that class. So, I'd look at doing something like (completely off the cuff):

package foo;

my $foo_thing = foo->new;
&::register_plugin($foo_thing);

sub new { return bless {}, shift }
sub hello { print "Hello from foo!\n" }

1;

package main;
sub register_plugin {
    push @plugins, shift;
}

foreach $plugin (@plugins) {
    $plugin->hello;
}
[download]

Neat idea, btw.

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