The first match includes the second comma, so the second match will start at the third one. 

Before s///:     ,  ,  ,  ,  ,  ,  , 6
After 1st pass:  ,,|  ,  ,  ,  ,  , 6
After 2nd pass:  ,,  ,,|  ,  ,  , 6
After 3rd pass:  ,,  ,,  ,,|  , 6

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The | indicates where a substition ended and from where the next pass will search.

Solution 1: Start from the begining instead of where the last match left off. 

1 while s{, +,}{,,};

[download]

Solution 2: Use a zero-width match to check for the "closing" comma. 

s{, +(?=,)}{,}g;

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In reply to Re: remove spaces only between commas by ikegami
in thread remove spaces only between commas by ktl

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