I solved:
add_action('save_post_product', 'update_post_meta_subito', 10, 3 );
function update_post_meta_subito($post_id){
$prova_termine="inzaghi";
$term_taxonomy_ids = wp_set_object_terms( $post_id, $prova_termine, 'pa_autore', true );
$thedata = Array(
'pa_autore'=>Array(
'name'=>'pa_autore',
'value'=>$prova_termine,
'is_visible' => '1',
'is_variation' => '1',
'is_taxonomy' => '1'
),
);
update_post_meta( $post_id, '_product_attributes',$thedata);
}