$1 and $2 are both defined when the match occurs.

That can't be as there is only one set of capturing parens. The second set of parens are a non-capturing lookahead.

This would probably do what you are trying to achieve: 

s/(?<=\d),(?=\d)/|/g;

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In reply to Re: regex match triggers "Use of unititialized value ..." warning by BrowserUk
in thread regex match triggers "Use of unititialized value ..." warning by mikeraz

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