I took a look at the MP3::Mplib, and to clarify what you have above, $mp3->set_v1tag() returns false if it fails. You can check the error of with $mp3->error().

if($mp3->set_v1tag({
                     year   => $args{year}  ,
                     artist => $args{artist},
                     album  => $args{album} ,
                     genre  => $args{genre} ,
                     title  => pop @mp3ary  ,
                   }, &ISO_8859_1)) {
    return mp3->get_v1tag;
} else {
    return $mp3->error();
}
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