Oops. Have overseen that. Thanks. Nevertheless, having a look on wire will clarify if the problem is below or above the transport layer and on which side it can be solved best.
Another random idea: The FTP commands are part of a print statement. I don't expect that this is a problem, but maybe ... it is worth to save the $ftp->dir() result in a variable and print it in a separate statement... just to exclude the (unlikely) possibility of some messed up I/O streams... ?