"I left" is simple past. I means it happened some time in the past. "I have left" is past perfect. It means the action happened in the past relative to the second verb, in this case "I'm going..." I would say the simple past is more common in America, where as the past perfect is more common among British English speakers.

The past perfect of "to leave" is "I had left". The present perfect is in fact "I have left". No mistake here. You are quite right. Perhaps the examples you gave do not quite exemplify the proper usage and meaning of the "present perfect". The "present perfect" is generally used to indicate completed events during an ongoing time period. Whereas the simple past indicates completed events during a time period in the past. Consider: Present perfect of "to see". "I have seen three movies this week." -> (completed = 3 movies, period = ongoing = this week). Simple past of "to see". "I saw three movies last week"-> (completed = 3 movies, period = completed = last week). The "present perfect" is generally used in conversations and implies a strong connection with the present. The focus is more on the events than on the time frame. E.g. "She has studied English." -> No time frame specified, but events are complete up to the implied present time.