The perl package is needed for building l10n translation files -- without it, the build will spit out a few errors but still complete, and the resulting netsurf-gtk binary ends up unreadable (all text labels display internal codenames). (Also FWIW, NetSurf and its deps currently lack an ~x86 keyword but it runs fine on that for me.)
I could not reproduce this problem with www-client/netsurf-3.3-r1, but www-client/netsurf-3.4 fails without it. So I add it there. commit 039de6a6b589d17c87932baa070b1ff276b79b48 Author: Michael Weber <xmw@gentoo.org> Date: Thu Mar 3 00:19:19 2016 +0100 www-client/netsurf: Add dev-perl/HTML-Parser to $DEPEND (thanks Anthony Parsons, bug 572920). Package-Manager: portage-2.2.27 www-client/netsurf/Manifest www-client/netsurf/netsurf-3.4.ebuild