"In perpetuum" means "forever" or "for an indefinite period of time". Antonyms for this expression could include "temporarily", "briefly", "intermittently", or "sporadically", among others. These antonyms suggest a limited time frame, a break in continuity, or an incomplete or occasional occurrence. For example, a project that is only temporary in nature might be described as having timelines that are not "in perpetuum". Similarly, a relationship that is on and off, or a job that is worked intermittently, could be considered antonyms to "in perpetuum". Overall, antonyms to "in perpetuum" connote limitation or incompleteness in some way.