[Haskell-beginners] How do I map a String and a IO String?

I want to replace lines with the content of a file if the line is a filename

{-# LANGUAGE LambdaCase #-} test = do putStrLn $ concatMap (\l -> l ++ "\n") $ map (\case l | (isPrefixOf "./" l) -> readFile l | otherwise -> l)

But since l is a String and readFile gives an IO String this doesn't work. How should I get around this? Kind regards, Jona