The OP hasn't actually made the ultimate goal clear -- my guess would be that a comma separated list of all fields is just an intermediate step while learning how to use DBI.

Starting with fetchrow_hashref and using a stub like this is probably a better place to begin:

foreach my $field_name (keys %{ $row_href }) {
  my $value = $row_href->{ $field_name };
  print "$field_name: $value \n";
}
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And if the order of the fields is important, you can replace keys %{ $row_href } with an array @field_names... typically, you have that information already, since you need it to build the SELECT.