You can also use Net::FTP built-in error reporting mechanisms: $ftp->ok(), $ftp->message(), $ftp->code(). All of those methods refer to the status of the last $ftp call that was made. (see Net::Cmd for info on the ok, message, and code methods that are common to most Net:: modules).

$ftp->some_command();
if($ftp->code()==425){
  # handle a "Can't open data connection." type error
}
#etc...
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The FTP error codes are detailed in RFC 959.