sub reader {
  foreach my $rows (@$data) {
    my %GenericData;
    @GenericData{ @rawFields } = @$rows; # NOTE added
    push @rawRecords, \%GenericData;
  }
return \@rawRecords;

##</code><code>##

my $rows = reader($mysqlFilename);

...in reader
my $json = encode_json($hashref, {
  lastname     => JSON_TYPE_STRING,
  firstname    => JSON_TYPE_STRING,
  age       => JSON_TYPE_INT}
);
...and I return $json which is the single record.