\G and ^ makes sense with /m (though I think I'd just match \n).

It also makes sense if you want to treat the first line specially. For example, /\G((?:^=|\+).*\n)/ extracts the first message from the following log file, line by line:

= This is a
+ very long
+ error message
= Message 2
= Message 3
+ is also 
+ very long
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