If you are installing from RPM, then you need to satisfy RPM that the dependency is installed. Installing modules through CPAN does not help because RPM does not know about them. Is the perl-Digest-SHA1 package installed? perl-Digest-SHA1-2.07-3 is included in Fedora Core 1 so install that.

yum helps a lot with installing packages and dependencies. yum depends "perl(Digest::SHA1)" will show what package provides that dependency. And yum install perl-Digest-SHA1 will fetch and install the package. Also, if you use a third-party repository with the packages you need then yum install spamassassin-tools will do it. dag.weers.com has spamassassin-tools-2.63-0.