If you have sailed on Carnival before, the only perk would be any sales that the World's Leading Cruise Line members might offer. For example, if Holland offered a special price for past guests, and you had sailed on Carnival (but not Holland) - you would qualify for the special price.
HOWEVER you are not considered a past guest, so would not be invited to the past guest party, or receive any past guest perks.
Carnival may be the parent company for Princess but Princess seems to resent it. On a recent cruise on the Diamond Princess, the cruise director one night made great poking jabs specifically at Carnival about its inferiority (to me, the cruise director poking fun at another cruise line is unprofessional and not funny - and I find him to be inferior.)
The following night the comedian entertainer produced a slide show comparing Carnival to Princess. The waitiers, the secruity, the food, the cabins, etc. IT went on and on and on. It would hav been funny if he had simply said "the Competition" - but instead, he specifically mentioned Carnival over and over and over again. I have gone on at least 6 different cruise lines and found them following their individual niche, to a degree - but it is unfair to say that things like security, cabins, food vary amongst the mainstream cruise lines, including Princess.
I have to say I have lost a huge amount of respect for Princess because of these - and other highly unprofessional things that occured on our cruise on the Diamond -things that I had never experienced before on the "cheap guys". Shame on Princess - they need to bring their noses out of the clouds, and start paying attention to the details. You can't say you're the best by words alone.
M. Fallone