The installation documentation is fine for Linux, as liblocale-gettext-perl is installed by the packages file. For MacPorts, it's easiest to add p5.12-locale-gettext to the list of packages to install.
But for Homebrew/other systems, installing Locale::gettext might not be trivial - the Makefile.PL checks that gettext is installed by writing and compiling a tiny C program. If your paths aren't set up correctly, this will fail and the module won't install. Locally, with gettext installed via MacPorts, but system CC/GCC, I had to run the following for it to work:
CC=/usr/bin/gcc C_INCLUDE_PATH=/opt/local/include LIBRARY_PATH=/opt/local/lib bin/cron-wrapper cpanm Locale::gettext
Someone with Homebrew should check whether simply having gettext installed with it is enough for it to install okay; if so, we could just add Locale::gettext to the carton.lock file and then deal with any other installation failures as they arise.
Have tested with homebrew, if you have done brew install gettext then Locale::gettext installs without issue if homebrew is installed in the default location. If it's installed elsewhere, I think you have to manually adjust Locale::gettext's Makefile as it just won't let you change LIBS, sigh.
brew install gettext
Have updated docs, hopefully with defaults it's okay, and if you don't have defaults you hopefully know what you're doing.