You have this line before reading any files hence you are not sending any data.
$req->content($xml);

Try moving it to after foreach loop

foreach my $file (@files){
  open FILE, $file or die $!;
  while (<FILE>){
    $xml .= $_;
  }
}
$req->content($xml);
[download]

Assuming you want to send all the data in one post.
poj