I found the source of my problem. I was adding the serialized data to a tar file via Archive::Tar->add_data. But I had missed the documentation in Archive::Tar that says:

Unicode strings need to be converted to UTF-8-encoded bytestrings before they are handed off to "add_data()":

So this change on the serializer

-    $tar->add_data($filename, $serialized_context, { type => Archive:
+:Tar::FILE });
+    $tar->add_data($filename, encode_utf8($serialized_context), { typ
+e => Archive::Tar::FILE });
[download]

and this change on the de-serializer

-    $self->serialized_blob($serialized_context);
+    $self->serialized_blob(decode_utf8($serialized_context));
[download]

make it all work as expected, and my original problem makes more sense now, though it's not much more than GIGO.