I was able to install the Ubuntu 18.04 package on my Ubuntu 17.10 box:
$ cat /etc/lsb-release
$ sudo apt install libfile-next-perl
$ wget https://launchpad.net/ubuntu/+archive/primary/+files/ack_2.18-2_all.deb
$ sudo dpkg -i ack_2.18-2_all.deb
$ ack --version
Running under Perl 5.26.0 at /usr/bin/perl
Copyright 2005-2017 Andy Lester.
This program is free software. You may modify or distribute it
under the terms of the Artistic License v2.0.
dpkg command fails for you, it's probably because you're missing some dependency that I already have. Try to
apt install any missing dependencies and then run the
dpkg line again.
I like this approach better than using CPAN, because APT/dpkg can still see this package is installed and will be able to upgrade it later once Ubuntu gets their build situation sorted out.
(Thanks Axel Beckert for posting link to the Launchpad bug report, which is where I got this idea.)