How many calories get absorbed when you cook something and then drain it?

While I'm about logging accurately for myself, I'd encourage you to keep this sort of thing in perspective: How many calories are we realistically talking about? In a lot of these cases, it just isn't going to be worth the bother arithmetically, IMO.

There's no way to be exactly exact on every food. One apple will be sweeter than the next, even at the same weight, so different calories. Food labels (in the US) need to be within X% of reality for calories (I think it's 20%), but they're not expected to be exact because that's impossible. Our specific bowl of soup maybe has more meat or beans and fewer tomatoes than someone else's, so the calories differ. Some entries will be over, some will be under, and it will tend to average out.

Moreover, if we're somewhat consistent in our eating habits and activity patterns, and log foods the same way every time, plus adjust calorie intake based on personal results rather than believing some so-called "calculator" (that really only estimates) . . . we'll have an average level of calorie error and it won't matter.

That's because we're adjusting our weight loss rate based on our results, which includes our standard level of logging error. (Even fitness trackers only give estimates, not measurements - they're just a little more personalized. All this stuff is estimates, from the food calories - no matter how carefully we weigh - to our daily calorie needs to exercise calories and more. Estimates!)

More specifically, to the question of arithmetic and perspective: A whole pint of fresh raspberries (about a third of a kg) has 156 calories. If you put them in water - briefly, I hope - how many calories end up in the water, for the portion you eat. Let's say I eat half that pint: 78 calories raw. If I lose half the calories in the water (unlikely) or guess it's that much but it's really fewer lost/more kept . . . we're fussing over 36.5 calories. If I'm losing at the slowest MFP-recognized rate, half a pound a week, that's a 250 calorie daily deficit. (Losing faster, deficit is bigger.) If I'm off by 36.5 calories, that's really trivial. It's not worth obsession.

(Maybe your raspberries are frozen/sugared - the same concept still applies: How much does a person need to worry about numerically small differences, when all is estimates?)

Over time, you'll learn ways to cook things that don't create as much uncertainty, if uncertainty bothers you. Maybe thaw your frozen raspberries in the microwave, or in the top of a double boiler, rather than dunking in water. Maybe cook your porridge in less milk so you needn't drain, or cook it in water and add hot milk to taste (yes, you'd still be draining water that might contain porridge dust calories).

But I'd encourage you not to let super-precision become an obsession. That can be a bad thing, psychologically. "Close enough" works fine for many people - not even weighing food works fine for some, even - just eyeballing.

I see people here occasionally who are so obsessed with calorie accuracy that they refuse to go to a restaurant, or to a friend's house for dinner, and cut themselves off from their social circle as a consequence. Worse, some seem to obsess into inability to cope with it, and seriously undereat to "make up for it".

It's good to be accurate, when it's practical. But extremes of accuracy seeking aren't necessary, and they really aren't healthy.

If you are ultimately eating all the raspberries just count all the calories for the raspberries in your log once. Doesn’t matter where. Most of the calories come from the solids you are consuming not from whatever taste leeches out in the form of diluted juice mixed into a pitcher of water.
I would be concerned about disordered eating if you are getting so obsessed with counting these seriously nonexistent calories and figuring out where and how to categorize them.