If you want more info on warnings and errors, try doing

use diagnostics;
[download]

-d $upload_dir or die "$upload_dir: $!";
-f $filename or die "$filename: $!";

# if we are here - we have valid directory and file
open (UPLOADFILE,"<$upload_dir/$filename") or die $!;
[download]

That would need to be:

-d $upload_dir or die "$upload_dir: $!";
-f "$upload_dir/$filename" or die "$upload_dir/$filename: $!";
[download]

Thank you. My error comes from the lack of fully written sample:

my $filename = "$upload_dir/$filename";

-d $upload_dir or die "$upload_dir: $!";
-f $filename   or die "$filename: $!";

# I'm not in the secret why those endless absolute
# filepaths are scattered almost always
[download]