sub hmac($key, $data, Callable &hash, $blocksize = 64 --> Buf)
Computes the HMAC of the passed information.
$data can either be Str or Blob objects; if they are Str they
will be encoded as ascii.
&hash needs to be a hash function that takes and returns a Blob or Buf. If
it operates on or returns a Str, it will not work. (The md5, sha1, sha256 functions
from Digest work well, as in the example above)
$blocksize is the block size of the hash function. 64 is the default, and
is correct for at least md5, sha1, sha256.