A few things:
Are you sure this worked:
ole_excel.WorkBooks.Open(l_str_dir)
If it didn't, then the worksheet you're working with doesn't have a name yet, and without a name the Save function wouldn't work. You would need to use SaveAs instead.
Are you actually starting from an existing file? If not, just do this instead:
ole_excel.workbooks.Add()
You would then have to set the filename through SaveAs.