A successful capture should not be undef. Whether it's documented or not, $1, etc are intended to be available to (?{}) and (??{}) blocks, so they must be set ASAP.

1. \w+ matches "ab"
2. () sets $2
3. Print $2
4. Second at \w+ fails to match: backtrack
5. \w+ matches "a"
6. () sets $2 (This isn't happening)
7. Print $2
8. \w+ matches "b"
9. () sets $2
10. Print $2

I do not see a compelling argument for "it should be the last thing matched by those 'physical' parentheses" over my proposal.

If that's true, then you would expect either of

$2=ab  # Only thing matched
$2=a   # Only thing matched
$2=a   # First thing matched
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and

$2=ab  # Only thing matched
$2=a   # Only thing matched
$2=b   # Last thing matched
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and the code is still buggy as it produces neither.

Clearer bug demonstration:

$ perl -e'"ab" =~ /((\w+)(?{print defined $^N ? "\$^N=$^N\n" : "\$^N n
+ot defined\n"})){2}/;'
$^N=ab
$^N not defined
$^N=b
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