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s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;

s/(
   ^[-+]?            # beginning of number.
...
   \G\d{3}           # after the last group, get three digits
   (?=\d)            # but they have to have more digits after them.
)/$1,/xg;