[Delvie]

Harvesting - There is an example by one of the Devs in one of the E3 reports that talks about if you will even notice a resource. It will depend on how fast you're travelling (that effects perception) and something else - I'll look for the specific report and post a link later.

Crafting - I'm not sure I've read anything 'official' about perception in crafting - the big thing there appears to be experimentation allowing you to learn new recipes - and Silius has said that he doesn't expect spoiler sites to help a lot with those recipes (I'm assuming then that there's an element of chance in what triggers the learning of recipes this way). There's been a lot of speculation that perception might play a part by watching other master craftsman though.

Diplomacy - Again I don't remember anything 'official' about perception here but I can just imagine that your parlaying away with the court jester and they pull a new technique on you, not only successfully resisting your efforts but almost convincing you that their way is better. The higher your perception the better your chances to learn the new technique.