Based on the context, I would say that the difference is whether or not you still work for the company. If you no longer work for the company, then you learned something there. If the timeframe of when you worked at the company is not clear or implied explicitly or implicitly, then you would speak of things you have learned while there.
From Wikipedia's entry on present perfect:
The present perfect in English is used chiefly for completed past actions or events, when no particular past time frame is specified or implied for them (it is understood that it is the present result of the events that is significant, rather than their actual occurrence). When a past time frame (a point of time in the past, or period of time which ended in the past) is specified for the event, explicitly or implicitly, the simple past is used rather than the present perfect.
It can also be used for ongoing or habitual situations continuing up to the present time (and not necessarily completed), particularly in describing for how long or since when something has been the case. In this case the present perfect progressive form is often used, if a continuing action is being described.