You probably have both 5.8.7 and 5.8.8 installed.
Do a perl -v and see what you get.
De-install perl5.8.7 (which, I believe, isn't installed by default, but is the version in the ports collection, so something else probably installed it).
You should be able to do this using "pkg_delete perl-5.8.7"
Then, update your ports tree using cvsup (I usually install cvsup-without-gui via a pkg_add to the FreeBSD ftp site).
Now, install perl-5.8.8 from the ports collection.
I've found that if you don't install 5.8.8 from the ports collection and try to do it from source, Plesk has problems finding files it needs.
Oh, and if it were me (but I'm anal), I'd blow the entire machine out and start from scratch. And I'd do the following:
Install FreeBSD 6.0
Rebuild Kernel
grab cvsup-without-gui from packages.
update ports collection
install perl-5.8.8
install latest stable openssl from ports
install whatever else you want
install Plesk.
But that's me.