echo "perl-core/Switch ~amd64" >> /etc/portage/package.keywords
To build puttygen only without X11 (ebuild currently from our overlay, see [https://bugs.gentoo.org/show_bug.cgi?id=482816 Gentoo Bug #482816]):
echo "net-misc/putty ~amd64" >> /etc/portage/package.keywords
echo "net-misc/putty -gtk" >> /etc/portage/package.use