type the following :

for the current official version :
ACCEPT_KEYWORDS="~x86" emerge piwi

or, if you want the bleeding edge, i kept some prelude-cvs ebuild which were removed from portage for you :
cd Doc/Gentoo ; ACCEPT_KEYWORDS="~x86" USE="ssl doc" emerge libprelude-cvs-0.8.ebuild prelude-manager-cvs-0.8.ebuild prelude-nids-cvs-0.8.ebuild prelude-lml-cvs-0.8.ebuild
cd Doc/Gentoo ; ACCEPT_KEYWORDS="~x86" USE="ssl doc" emerge piwi-cvs-0.8.ebuild



then follow what is written at the end of installation

this include adding this to apache configuration :
(/etc/apache2/conf/apache2.conf)

<IfDefine PIWI>

	<Directory "/home/httpd/htdocs/piwi">
		Options ExecCGI FollowSymLinks
		AllowOverride All
		DirectoryIndex index.pl
	</Directory>

	<IfDefine MOD_PERL>
		PerlModule Apache::DBI
		<Files *.pl>
			SetHandler perl-script
			PerlHandler Apache::PerlRun
			PerlSendHeader On
		</Files>
	</IfDefine>

	<IfDefine !MOD_PERL>
		AddHandler cgi-script .pl
	</IfDefine>

</IfDefine>

and in "/etc/conf.d/apache2" add "-D PIWI" to APACHE2_OPTS string
