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The best place to read up about Perl 6 regexes is the specification itself.
While I suspect this has something to do with '\0' terminated strings in C, I don't fully understand what's happening.
No, it's not anything to do with C string terminators.
The problem with your previous version was that you were matching an optional comma at the end of each field and then replacing it with a definite "\037" every time. So, for the last field in each record (which, of course, isn't followed by a comma), your were nevertheless appending an unwanted "\037".
The global substitution would then loop one last time, matching a final zero-character field (because of the (?<a>[^,]*) alternative, which can match nothing). The substitution on that empty field then causes a second unnecessary "\037" to be appended.
You could fix that by rewriting your original version something like this:
This version still matches the optional comma each time, but now only appends a "\037" if there actually was a comma. Which means there are no extras to remove, once the line is complete.
Note that I also removed the chomp and replaced it with an explicit substitution of the trailing newline. I felt that this highlights the transformation more clearly than did your clever (but subtle and "at-a-distance") use of $\.