Have you tried $ftp->ls() without the UNIX-specific parameters "-Al"?

$ftp->ls expects a directory as the argument, or when empty, will list current directory. That you can use command line swithes could be called a bug, or a feature, depending on who you ask. :) Point is, that the UNIX ftp server either asks the shell for ls "whatever you said", or is aware of such switches. (That is, of course, assuming that you have those switches for a reason, and that they produce different results?) Serv-U most likely isn't, although you could, for fun, try to enter Windows/DOS-specific switches and see what happens.

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You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.