I would prefer:

sub wait_for_files {
   my $files = shift;

   my @not_changed_yet = keys %$files;
   while (@not_changed_yet) {
     @not_changed_yet = grep { ! -e } @not_changed_yet;
     sleep(1) if @not_changed_yet;
   } 
}
[download]

You should at least sleep(1) and maybe longer.

Also, I am testing only the files that are still left.