According to the the law of Quo Primum, the law itself remains in effect "in perpetuity."
If you say it can be changed, then pope St. Pius V had no authority to make the law remain in effect forever. If he did not have that authority, then obviously no one told him he couldn't do that. smh
If you say "in perpetuity" means no
Pope could make changes to the missal (that's what Pius V promulgated in perpetuity), then Pius V himself violated the law he enacted, since he changed the missal a few years after promulgating Quo Primum by adding the Feast of the Holy Rosary to the missal. And Pius XII made huge revisions to the Missal when he revised Holy Week.
So, either "in perpetuity" doesn't mean what you think it means, or Pius V and Pius XII (as well as other Popes) have done what the law forbids and have incurred the wrath of Almighty God and the Blessed Apostles Peter and Paul.
So, explain why you believe in perpetuity, in the context of a law established by a Pope, means what you think it means. If your answer is that you are simply applying the word based on your understanding of what it mean, that's not good enough. You need to show that when the Church uses the phrase in the context of a liturgical law it means no future Pope can change or abrogate it.