Be aware that inheritance is not determined at compile-time. When you do $obj->method(ARGS), what you're actually doing is $obj->can('method')->(ARGS). can() looks something like this:

sub can {
    my ($self, $meth) = @_;
    my $class = ref $self || $self;
    my $subref = *{$class . '::' . $meth}{CODE};
    return $subref if defined $subref;
    for (@{$class . '::ISA'}) {
        $subref = $_->can($meth);
        return $subref if defined $subref;
    }
    return undef;
}
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There's a lot of error-handling I've skipped, both in the definition of can() and the "expanded code" ($obj->can('method')->(ARGS)). The point is, you can modify @ISA at any time; there is no compile-time method resolution.