At a guess, I think you'd do something like this:

#!/usr/bin/perl

use DBI;
use JSON;

...
$sth->execute();
while (my $hr = $sth->fetchrow_hashref) {
    push @array, $hr;
}

my $json = JSON->new;
print $json->objToJson(@array);
[download]

Caveat emptor, etc.