sub __PACKAGE__::NewWay::new {
    my $class= shift;
    API->use_the_new_way;
    return bless [], $class;
}
sub __PACKAGE__::NewWay::DESTROY {
    API->use_the_old_way;
}
sub AUTOLOAD {
    ...
    my $newway= __PACKAGE__::NewWay->new();
    goto \&$func;
    # or, if you want to keep AUTOLOAD in the stack trace:
    # return &$func( @_ );
}