MariaDB: Procedure using dynamic outfile name

CREATE DEFINER=`wendelp`@`%` PROCEDURE `test`()
    LANGUAGE SQL
    NOT DETERMINISTIC
    CONTAINS SQL
    SQL SECURITY DEFINER
    COMMENT ''
BEGIN
declare var_filename,var_titel, var_endung,var_dok varchar(100);
declare var_dokument longtext;
declare var_done int default false;

declare export_cur cursor for
select distinct
    doc.Titel, doc.Dokument, doc.Dateiendung, vov.Key_DOK
    from VOV_MED vov
    join DOK_MED doc
        on doc.Key_DOK = vov.Key_DOK
    where vov.Typ in (1,7,9,11,12,13,14,15,16,17,18,19)
        and vov.Key_DOK > 0;

declare continue handler for not found set var_done= true;

open export_cur;
read_loop:loop
    fetch export_cur into var_titel,var_dokument,var_endung,var_dok;
        if var_done then
            leave read_loop;
        end if;

        set @sql = concat('select ''', var_dokument, ''' into outfile ''/opt/abdamed/', replace(var_titel,'/','_'),'_',var_dok,'.txt','''' );
        prepare psql from @sql;

        execute psql;


end loop;
close export_cur;



END