If the action was quite recent, you can use "have." For example, the person reviewing your accounting books looks up and says "I have come across an entry I don't understand."
Or, the action was some time in the past, but the timing is not important.
Person A: I have never seen any examples of species of the Grammaricae Geekius Person B: Oh, I happened to have come across on
If you refer clearly to a specific, finished past event, I'd prefer "I happened to come across some interesting additional information yesterday."
To speak more generally (i.e. when you came across the information is not important) with relevance to the present, then I might say this: "I happen to h