Nevermind, I found a similar thread in the archives and 'rm -rf /usr/lib/fwknop' before re-installing fixed it.
Slackware 12.2 was released yesterday and being a minor release I didn't think an upgrade would cause many problems, but it broke fwknop!
The error message I get is:/usr/bin/perl: symbol lookup error: /usr/lib/fwknop/i486-linux-thread-multi/auto/Digest/SHA/SHA.so: undefined symbol: Perl_Tstack_sp_ptr
Both 'fwknop' and 'fwknopd' generate this error message.
Can anyone give me any advice on how to debug this and get it working again?