No meaningful tests.
'file_md5', 'url_md5', etc. reads entire files into memory rather than using Digest::MD5's addfile() function or reading the file in chunks and using Digest::MD5's add(). (Strangely, however, addpath() does not have this problem.)
'adddir' relies on the order of keys in a hash to produce consistent results, which is broken since 'identical' directories may be read in different orders and because of hash randomization.
Passing binmode argument to 'file_md5*' does not actually do anything.
Documentation uses 'Digest::Md5' where it means 'Digest::MD5'.
Probably should subclass Digest::MD5, not add methods in its namespace.
Probably should have BINMODE, etc. be settable as per-object properties in its OO extensions.
Using readline() with a default $/ to read chunks of input is a poor choice for binary files.