For arrays of the same type, there's a function where you don't need to know the number:

my $typehint_persons = json_type_arrayof($typehint_person);
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Learning how x works is cool, but you don't need it here, see Re^2: Is there a way to make a JSON out of multiple records from MySQL?.
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Actually have a lot of debugging to do. As I am getting a lot of the following errors, AND in addition to what appears to be a valid JSON output

For each record, I get:

Use of uninitialized value in subroutine entry..

AND a complete JSON output

If I lacked clarity, in my call to retrieve the records I am using:

my $data = $dbHandle->fetchall_arrayref();

Then I push each hash reference onto an array

    foreach my $rows (@$data) {
            my %GenericData;
            @GenericData{ @rawFields } = @$rows; 

            push @rawRecords, \%GenericData;
        }
     return \@rawRecords;

     ##################
     #  which is returned to the calling function as $data #
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Then I pass the reference $data to:

makeJSONToExport($data);

sub makeJSONToExport {
    my ($hashref) = @_;

    my $typehint_raw =  {
            age        => JSON_TYPE_INT,
            lastname      => JSON_TYPE_STRING,
            firstname          => JSON_TYPE_STRING};

    my $typehint_raws = [ ($typehint_raw) x  @$hashref];
    my $json = encode_json($hashref, $typehint_raws);
    # my $json = encode_json($hashref, );
    return $json;
}
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While I'm glad I'm getting a legit JSON, I am concerned about the error msg

Use of uninitialized value in subroutine entry..

You should see at least a line number associated with this warning (not error) message, and it should be fairly easy to zero in on the argument the subroutine is expecting and not getting.

my $typehint_raws = [ ($typehint_raw) x  @$hashref];

If @$hashref is indeed a hash reference, I don't see how the code runs beyond that statement.

Win8 Strawberry 5.8.9.5 (32)  Wed 05/11/2022 21:07:27
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

my $hr = { qw(a b  c d) };

print "got here \n";
print @$hr, "\n";
# print %$hr, "\n";  # works
print "and here \n";

^Z
got here
Not an ARRAY reference at - line 8.
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---

Give a man a fish:  <%-{-{-{-<

To be clear,I left the line number out because it means nothing, but it is definitely on the line:

my $json = encode_json($arrayref, $typehint_raws);

Using print Dumper I verify that:
*$typehint_raws becomes one empty array for each record found
*$typehint_raw is a hash which has each column and and whether it's an int or string

Once again, everything works, but each call to the above line returns that error.

Here's the code I use to retrieve each record....

$data = readRaw(table name);

sub readRaw {
    my ($rawfilename) = @_;

    my $dbCommand = 'select '.join ',', @rawFields;
    $dbCommand .= qq^ from $rawfilename;^;

    $dbHandle=$db->prepare($dbCommand);
    $dbHandle->execute();

    my $data = $dbHandle->fetchall_arrayref();
    $dbHandle->finish;

    foreach my $rows (@$data) {
            my %GenericData;
            @GenericData{ @rawFields } = @$rows; # NOTE added

            push @rawRecords, \%GenericData;
        }

     return \@rawRecords;
}
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So I have an array of arrays, but each array holds the reference to a hash for each record read

And $data is passed to this function

sub makeJSONToExport {
    my ($arrayref) = @_;

    my $typehint_raw =  { age        => JSON_TYPE_INT,
            lastname      => JSON_TYPE_STRING,
            firstname          => JSON_TYPE_STRING};


    my $typehint_raws = [ ($typehint_raw) x  @$arrayref];
    my $json = encode_json($arrayref, $typehint_raws);
    return $json;
}
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