Try to put this into your class A:

use Class::ISA;
sub method_class {
    my ($self, $method_name) = @_;
    my $coderef;
    my $class;
    foreach (Class::ISA::self_and_super_path(ref($self)||$self)) {
        my $coderef_cur = $_->can($method_name);
        last unless defined $coderef_cur;
        last if defined $coderef and $coderef ne $coderef_cur;
        $coderef = $coderef_cur;
        $class = $_;
    }
    return $class;
}
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...it is not very tested, and I am not sure about autoloading, but I suppose this is what you seek. BUT, remember Merlyn's words, class would never care about method actually launched. Update: I did not tested multiple inheritance.