+----------------------------- In the scalar $s (interpreted as a string),
 |       +--------------------- if you find an _ (underscore)
 |       |   +----------------- that's followed by 0 or more of anything but an _ (underscore) 
 |       |   |                  and then the end-of-string,    
 |       |   |             + -- replace it with a , (comma).
 |       |   |             |
 V       V   V             V
$s =~ s{ _ (?= [^_]* \z) }{,}xms;