Hi again :) (I really like your questions, they make me think even as a native speaker) From my knowledge, perennial is similar to having many reoccurring cycles, like a repetition of something. The most common place that perennial is used is when talking about flowers, perennial describes a plant that lives for several years, through seasons...a repetition. Perpetual, like perennial, describes something that lasts a very long time, however perpetual has a more constant meaning to it, like the sun, it is constantly shining and doesn't have cycles, its repetitions are constant and continual. "The perennial flower is bathed in perpetual sunlight." =p This may be because I am ignorant, but I have only very rarely heard perennial used outside of talking about plants. Perpetual is going to be the better choice between the two words in most sentences, perpetual has a positive tone and is usually associated with light, angels, or other religious symbols. I hope this helps, if you want more examples just ask.