After some discussion in the CB, let's look at the facts...

• $mp3->set_v1tag() returns true
• $mp3->error() is returning {album => 2 }
• $mp3->get_v1tag() returns an empty hashref

Looking at the doc's for MP3::Mplib, there are various constants used for the error codes, which are available in mplib.h. From this define,

#define MP_EFNF 2
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the modules saying that "album" is not a valid tag for v1. A look at the XS code for the module give the answer pretty quickly.

         if (strcmp(key, "ALBUM") == 0) {
            content = mp_assemble_text_content(val, enc);
            if ((err = mp_set_content(new_tag, MP_ALBUM, content)) != 
+0)
                sv_catpvf(mp3_lib_err, "%s\034%i", "ALBUM", err);
            continue;
        }
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Try switching your tags to uppercase and see what happens. The code is looking for all uppercase hash keys and there doesn't seem to be any code attempting to uppercase the hash keys at all.

OK, so the problem is not likely in the function shown above. What does the calling function get back?