Actually, Digest::MD5 works on OS X also:

$ perl -MDigest::Md5 -e ''
$ perl -MDigest::MD5 -e ''
$
[download]

The weird mix of "sometimes case-sensitive, sometimes case-insensitive" in OS X is one of my bigger pet peeves about it in theory, but, in practice, it's only bitten me once (when I created two files in the same directory whose names differed only in case and I attempted to delete one, but ended up deleting both).

Based on other responses by the OP, it's clear that he is using Windows, so your conclusion is correct, even if it's based on a data point which doesn't necessarily support it.