I'm curious what the RE gurus looking into the monastery will say. However, so far, it seems to me to be neither a bug nor a feature, but just an oddity that comes from an somewhat unfortunate [mis]use of /g.

As I understand it, /g is not a substitute for /sm - it is sort of an iterator that lets you steadily step through the matches in a string, if you need a step-by-step, match-by-match approach. See it in conjunction with pos.

Successive, iterated substitution - which /g seem to imply, are clearly weird: the intermediate string resulting after every substitution is something different from both the initial string as from the final result, and there is by no means any intent to use it for anything other then as 'something' unfinished.

Think about something like

my $test = 'AAAA';
my $x1 = $1 if $test =~ s/A/AA/g;
my $x2 = $1 if $test =~ s/A/AA/g;
...
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Would you expect any intermediate results ?! I think not, and so I wouldn't expect anything from $1,$2,... after a s//g, similar to like I don't really trust for example a for-loop control variable to be something I can rely on once the loop has finished. I remain curious about what others think/know about it.