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After doing that DB_File can be installed from CPAN.
Mac OS also tends to do unusual things with their Perl installation. To avoid problems with that, install perl yourself either using homebrew or perlbrew. With the latter you can use perlbrew to install cpanm, then use cpanm to install DB_File and other modules that are required to build SpamAssassin.
This discussion post implies that it is not possible to get the system perl to use a different Berkeley DBThe system Perl will not use the new install of Berkeley DB, continuing to use the one baked into libSystem.B.dylib. It will be necessary to install Perl yourself, such as by using Macports, homebrew, or perlbrew, then installing DB_File from CPAN so that DB_File is built using the new berkeley-db.