The package consists of a single C source file with about 300 lines, which is certainly copyrightable. So some sort of license is needed. Definitely not "as-is".
Mail sent to upstream (hddled@very.puzzling.org).
Answer received. The author, Michael Chapman, has released hddled.c into the public domain: http://members.optusnet.com.au/foonly/whirlpool/code/hddled.c
Bumped to 0.3 (which is unchanged from 0.2 except for the license).
Thanks