Ah this does work. I should have check the newsgroups a bit more.

I wish the perldoc for Crypt::DSA had mentioned that there is a write function.

The Crypt::DSA doc says

$signature = $dsa->sign(%arg) Signs a message (or the digest of a message) using the private portion of the DSA key and returns the signature.

The signature is a hash reference with two keys: s and r.

%arg can include:

Digest

A digest to be signed. The digest should be 20 bytes in length or less. You must provide either this argument or Message (see below).

Key

The Crypt::DSA::Key object with which the signature will be generated. Should contain a private key attribute (priv_key). This argument is required.

write (what zentara used), is a method of $key, a Crypt::DSA::Key object, so it makes sense that its documented in Crypt::DSA::Key.