Hi kenny1999 They don't carry a different meaning. But I would write the second one because there seems to be no need to add a comma. If you say it out loud though, the pause created by the comma would add to the effect that you were astonished.
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kenny1999A. I was astonished when I saw Peter, quarrelling with his neighbour.This is catenative see. No comma!
kenny1999B. Quarrelling with his neighbour, I was astonished when I saw PeterAs you say, this one has a different meaning. Here, you were quarrelling with Peter's neighbour.