The source code comes from Mail::SpamAssassin-2.55.tar.gz, available from
http://www.spamassassin.org/.  This package is a stopgap until the Debian
spamassassin package provides libspamc.  The original COPYRIGHT file from
SpamAssassin says this:

The code in the files of the SpamAssassin distribution are Copyright
2000-2002 Justin Mason and others, unless specified otherwise in that
particular file.

All files in the SpamAssassin distribution fall under the same terms
as Perl itself, as described in the file named "License".

Debianiazation by Mark Wooding.