I lived there, and in Brunei.
Fishing...hmmmmm..what do you fancy?
Most of the fishing is for predatory fish, and strangely enough, not many of them are Asian! You can catch Arawana, Toman, Snakeheads, Peacock Bass, Red Mahseer, Hampala Barbs and so on. The locals, if not using an old "Milo" can with line wrapped around it and a chunk of fish as bait, are very American orintated, using baitcasting gear and soft plastics or lures.
There are several "fishing ponds" in KL itself, along with rivers and "sungais". These are unasuming drainage ditches that hold Gouramis, Siamese Fighters, Climbing Perch and many other types of "tropical fish tank" species. Its rather funny catching Gouramis in what amounts to a sewer when back in the UK they are kept in pristine tanks.
There are even commercial fisheries available for huge carp!
You will be pleased to know there are 5 or 6 good tackle shops there too.
You are also only 20-30 miles from the sea. (Malacca Straits) Here you can fill your boots with all sorts of fish. When I lived out there Pirates were an issue...not sure if they still are? You can target Wahoo, GT, Cobia, Barracuda, Marlin, Amberjacks and lots of sharks.
Sandy can advise on the fly gear to take as it is similer fishing to Brunei, but a 6 and a 9 weight salt water set up would do for most fish I think. Remember to use tropical fly lines as the heat can be extreme, especially inland in some of the large rain forest lagoons where there is no wind.
All I can say is you lucky lucky sod!