Using only look-arounds and no captures. Express the conditions: insert '0' at every point that is:

• not after a digit; and
• before a solitary digit.

perl -wMstrict -le
"my @strs = (
   '1,2,3,4', '8,9,10,11', '7,8,9,10', '12,13,14,15',
   '1,23,456', '123,45,6',
   );
 ;;
 my $not_after_digit       = qr{ (?<! \d)        }xms;
 my $before_solitary_digit = qr{ (?= \d (?! \d)) }xms;
 ;;
 for my $s (@strs) {
   printf qq{'$s' -> }, $s;
   $s =~ s{ $not_after_digit $before_solitary_digit }{0}xmsg;
   print qq{'$s'};
   }
"
'1,2,3,4' -> '01,02,03,04'
'8,9,10,11' -> '08,09,10,11'
'7,8,9,10' -> '07,08,09,10'
'12,13,14,15' -> '12,13,14,15'
'1,23,456' -> '01,23,456'
'123,45,6' -> '123,45,06'
[download]