A more thorough explanation:

in perpetuum

Forever; always happening.

• The land was given to the park in perpetuum, meaning it will always be a park and cannot be used for anything else.
• The artist's work will be remembered in perpetuum, as it has had a lasting impact on the art world.

These examples show that when something is said to be "in perpetuum," it means that it will last forever or always be happening. The first example shows that the land will always be a park and cannot be changed. The second example shows that the artist's work will always be remembered and have an impact on the art world.