initialization
execute an SQL statement
while (my $aref=$sth->fetchrow_arrayref) {
    ... write to spreadsheet ...
    prepare & execute an SQL statement
    while (my $aref=$sth->fetchrow_arrayref) {
        ... write to spreadsheet ...
        prepare & execute an SQL statement
        while (my $aref=$sth->fetchrow_arrayref) {
            ... write to spreadsheet ...
            prepare & execute an SQL statement
            while (...) {
               <<< same thing for several more levels
            }
        }
    }
}