Sort of like this...

sub direc
  {
  my $dir = "/my_dir";
  my @dirs = map m[/(\d{8})\z], glob "/$my_dir/*";
  my $latest = (sort { $a % 1e4 <=> $b % 1e4 } sort @dirs)[-1];
  [ glob "/my_dir/$latest/*.txt" ];
  }
[download]

(untested)