I was just struggling for an hour with the meaning of
> It might help to quote what you want to match.

when the following arrived, thank goodness:
< SpamAssassin joins all mail header fields with the same header name
< into one multine string of header field bodies,

Ah, no wonder $ matched but ^ didn't.
Delivered-To: "jidanni@jidanni.org
mailing list users@spamassassin.apache.org"

< so whenever one uses a ^ or $ in a pattern, one is almost obliged to
< append a /m flag, otherwise one risks being at a mercy of malicious
< senders... Depending on a situation, this can be a security risk.

Sure wish Mail::SpamAssassin::Conf would mention all this where it
discusses headers. Also it should mention loss of \Q\E.