package Plugin;
...
sub process {
   my $self = shift;
   my $foo = shift;

   $foo->FooMethod();

   ...
}

sub processes {
    return qw/Foo/; # this module's process method likes objects which isa Foo.
}

##</code><code>##

sub process {
    my $self = shift;
    my $obj = shift;
    my @args = @_;

    DISPATCH: foreach my $plugin ($self->host->stack("process") { # the stack of plugins that ->process
        foreach my $class ($plugin->processes){
            next unless $obj->isa($class);
            return $self->host->specific($plugin)->process($obj, $args);
        }
    }
}

##</code><code>##

package Plugin;

sub process : expects(FooClass) {
    my $self = shift;
    my $foo = shift;
}