AIX

user ijkl password
cd /home/yyy
site [[ -s xxx.dat ]] && append xxx.dat xxx.dat
site [[ ! -s xxx.dat ]] && put xxx.dat xxx.dat
close
quit

#!/usr/bin/perl

use warnings;
use strict;
use Net::FTP;

my ($host, $user, $pass) = qw(<host> <user> <pass>);

my $_file = 'xxx.dat';

my $ftp = Net::FTP->new($host, Debug => 0, Timeout => 240)
  or die "Cannot connect to $host: $@\n";


$ftp->login($user, $pass)
  or die "Cannot login ", $ftp->message;

eval {
  $ftp->size($_file)
   };

$@ ? $ftp->put($_file) : $ftp->append($_file, $_file);

$ftp->quit 
  or die 'Failed to disconnect ', $ftp->message;