First, for clarity, you want to indent the code within each subroutine, e.g.

sub sendFile {
       # application object
       my $self = shift;

      # load template files
      my $tmpl = $self->param('info' => $self->load_tmpl('info.phtml')
+);
...
}
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Re: your question of where the bug is, I think it's here:

    # get the filename
    my $filename = "dir+filename";

    # check if file exist
    if(-e $filename){
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1. Is the actual path of the file you want to send really "dir+filename"? Or was that intended to be a note to yourself to assign it something like "/home/boboson/media/mp3.mp3"?
2. You may want to replace your -e $filename test with -f $filename, as the former tests only for existence, the latter tests if it's an actual plain file. See the perldoc for more info and other tests you can run.