@output should be an array of hashrefs.

while ( my $row = $sth->fetchrow_hashref ) {
    my %data;
    $data{ col1 } = 'bar';

    push @output, \%data;
}
[download]

Then converting it to JSON will make it look something like (i've added some formatting):

{
  "myData" : [
    {
      "col1" : "bar"
    },
    {
      "col1" : "bar"
    },
    # etc ...
  ]
}
[download]

HTH.