From the documentation: https://laravel.com/docs/7.x/database#running-queries

The select method will always return an array of results. Each result within the array will be a PHP stdClass object, allowing you to access the values of the results:

You need something like this to get the result.

DB::select('call generateCugPlanReport(:prefixName, @cugPlanReportID)', [ "prefixName" => $data["prefixName"] ]);

$cugPlanReportID = null;
$selectResult = $DB::select('SELECT @planReportID AS planReportID');

if (!empty($selectResult) && isset($selectResult[0]->planReportID)) {
    // we have a result
    $cugPlanReportID = $selectResult[0]->planReportID;
}