If the fields are prefixed with length bytes, then you can prevent 'a*' (or 'A*' or 'Z*') from consuming the rest of the string by telling format that the length byte is there.

$x = pack 'c/a*N', '12345123451234512345', 99999;

print for unpack 'c/a* N', $x;
12345123451234512345
99999
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The downside is that the length byte is then consumed. To workaround that, you have to unpack the length byte twice.

print for unpack 'cXc/a* N', $x;
20
12345123451234512345
99999
[download]

However, I don't see how this helps the OP as his data has a 'type' byte but no 'length' byte.