It should work per: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=743280 But I couldn't try as I see that something must be missing in ebuild causing it to still get linked against 2.8 slot even switching the version in ebuild :/ (some missing configure option maybe?)
wxperl gets built against whatever wxGTK version Alien-wxWidgets was built with unfortunately.
[master da34b6a] dev-perl/wxperl: Support wxGTK:3.0 (#563340) 1 file changed, 46 insertions(+) create mode 100644 dev-perl/wxperl/wxperl-0.992.700-r1.ebuild