Are there plants (for instance berries) which lead to dehydration if eaten?
I'd wager that for most people in practically any reasonable situation you would encounter in the outdoors other sources of dehydration would be a bigger factor than dehydration via eating berries. Perhaps if you were laying down with a huge mound of fairly dry berries, wearing a mask that let you eat but minimized water loss due to water vapor in your breath, in a constant comfortable temperature to avoid any sweating, and constantly eating berries then berry induced dehydration might be a factor. Even then I think you might lose more water in your feces than digestion. There are so many other bigger ways that you lose water than digestion that I don't think this is the factor that will be the most important thing to control.
I would even go as far to say that if you're stranded in a hot desert it might be a good idea to eat dry salty crackers or some salty jerky. This is because the salt will help you retain whatever water you can drink, and help make up for all the electrolytes you're sweating out. Food driven dehydration wouldn't be my biggest concern personally.
All that being said... imsodin mentioned this in a comment:
The obvious answer is yes, there is plants that can cause diarrhea and that is the ultimate way to get dehydrated
This is very true. If you eat a berry that caused diarrhea then the diarrhea certainly cause dehydration. According to the Livestrong Foundation:
Many fruits have high fiber content that can potentially cause you to have diarrhea, although no medical organizations recognize certain fruits as being particularly likely to cause diarrhea.
That isn't very promising because it says that medical organizations basically say no. Then in another part of their site they say:
Blackberries are a healthy snack or dessert, but if you experience diarrhea after eating them, blackberries might not be a good food choice for you. Diarrhea can occur for several reasons after you eat blackberries. The most common causes of diarrhea from blackberries include salicylate sensitivity, food allergy and food poisoning.
This leads me to believe that some people might have a food allergy to something in a berry, like salicylate sensitivity, which can cause predicable diarrhea. Interestingly enough WebMD says that blackberries can be used to treat diarrhea, so I guess this berry is a bit of a mixed bag.
Regardless, the moral of the story is if you are allergic to a component in a berry, and that allergy causes diarrhea, then eating berries might lead to dehydration via diarrhea. All in all I'd say that unless you are one of the few with a pretty specific allergy you're more likely to get diarrhea from drinking directly from a stream via giardia.
Would it be worth to eat them if you have enough drinkable water?
Probably not. Diarrhea isn't fun and you can go for a long time without eating. If you know eating blackberries gives you diarrhea, I'd recommend finding something else to eat.
Overall, as I'm sure you know, this is more of a game mechanic than something you'll likely encounter in a true survival situation, unless you have the same food allergies as your in-game character. The more I think about it that might be an interesting game mechanic to allow a player to specify their allergies so different things become deadly or harmless.