What is the difference between in tense between "I learned that song" and "I have learned that song."?
Past Simple tense I learned that song informs only about past event but does not indicate if the situation has any impact on presence.
Present Perfect tense I have learned that song informs not only about past event of learning, but also shows that the act of learning the song has its influence on present situation i.e. I know the song (probably lyrics) now.
Another often used example is:
I lost my keys vs I have lost my keys.
The first sentence informs only about past event, so we don't know if the speaker has the keys now or not. He might have found them in the time between loosing and now. The sentence informs about past event but it does not tell anything about what happened later.
The second sentence informs that the act of loosing keys has its influence on present situation. The speaker does not have his keys now. This is shown by using Present Perfect tense.