How to fix a locale setting warning from Perl

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LANGUAGE = (unset),
    LC_ALL = (unset),
    LANG = "en_US.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

Adding the following to /etc/environment fixed the problem for me on Debian and Ubuntu (of course, modify to match the locale you want to use):

LANGUAGE=en_US.UTF-8
LC_ALL=en_US.UTF-8
LANG=en_US.UTF-8
LC_CTYPE=en_US.UTF-8