c:\@Work\Perl>perl -wMstrict -le
"use 5.010;
 ;;
 my $word = qr{ [^,]+ }xms;
 my $comma_between_2_words = qr{ $word \K , (?= $word) }xms;
 ;;
 for my $string (@ARGV) {
   print '';
   printf 'question a: ';
   my $match = $string =~ $comma_between_2_words;
   print qq{'$string':}, $match ? '' : ' NO', ' match';
   ;;
   printf 'question b: ';
   $match = $string =~ m{ , .* , }xms;
   print qq{'$string':}, $match ? '' : ' NO', ' match';
   }
"  ",,"  ",x,"  "x,x"  ","  "x,y,z"

question a: ',,': NO match
question b: ',,': match

question a: ',x,': NO match
question b: ',x,': match

question a: 'x,x': match
question b: 'x,x': NO match

question a: ',': NO match
question b: ',': NO match

question a: 'x,y,z': match
question b: 'x,y,z': match