my $ftp = Net::FTP->new($_[0], Debug => 1) or die "Cannot connect to $
+_[0]";
$ftp->login($_[1],$_[2]) or die $login_failed_message;
...
$ftp->put($_[3]) or die "get failed ", $ftp->message;
[download]

Probably one of the many die()'s triggered? You could wrap the call to upload_file() with an eval block:

my $ftp_message = 'n/a';

eval {
  $ftp_message = upload_file($server_ip,$server_user,$server_pass,$fil
+e_to_upload,$os);
}; 

if ($@) { # catched an exception
   $ftp_message = "Error: $@";
   ...
} else {
   ...
}
[download]

Update: Oops, misread the question (but you still need to handle exceptions).

One possibility is to hook into the Net::Cmd::debug_print() method, which performs sending the debug messages to STDERR:

{ # temporarily change Net::Cmd::debug_print
  my $log_buffer;

  local *Net::Cmd::debug_print = sub {
           my ($self, $dir, $text) = @_;
           $log_buffer .= ($dir ? 'snd --> ' : 'rcv <-- ') . $text;
        };

  my $ftp = Net::FTP->new($_[0], Debug => 1) or die "Cannot connect to
+ $_[0]";
  ...
  $ftp->quit;

  print "Log:\n$log_buffer\n";
} # recovers previous behaviour of Net::Cmd::debug_print
[download]