Well, this idea isn't new. It has (of course) been discussed
on p5p. And was never implemented because noone could find
a solution to the problems you list. What are you going to
do with /.*;/, which basically asks you to find the last
semi-colon in the stream?
Nothing. Anybody asking for any *last* thing in a stream is asking for trouble! Streams are of infinite length by concept, which is something one should be aware of before trying to combine streams and regexes.
So in brief: /.*;/ should yield the whole stream in-memory. Which is exactly what the author of the regex asked for.