How to fix a locale setting warning from Perl

Your OS doesn't know about en_US.UTF-8.

You didn't mention a specific platform, but I can reproduce your problem:

% uname -a
OSF1 hunter2 V5.1 2650 alpha
% perl -e exit
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LC_ALL = (unset),
    LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

My guess is you used ssh to connect to this older host from a newer desktop machine. It's common for /etc/ssh/sshd_config to contain

AcceptEnv LANG LC_*

which allows clients to propagate the values of those environment variables into new sessions.

The warning gives you a hint about how to squelch it if you don't require the full-up locale:

% env LANG=C perl -e exit
%

or with Bash:

$ LANG=C perl -e exit
$ 

For a permanent fix, choose one of

1. On the older host, set the LANG environment variable in your shell's initialization file.
2. Modify your environment on the client side, e.g., rather than ssh hunter2, use the command LANG=C ssh hunter2.
3. If you have administrator rights, stop ssh from sending the environment variables by commenting out the SendEnv LANG LC_* line in the local /etc/ssh/ssh_config file. (Thanks to this answer. See Bug 1285 for OpenSSH for more.)

Here is how to solve it on Mac OS X v10.7 (Lion) or Cygwin (Windows 10):

Add the following lines to your bashrc or bash_profile file on the host machine:

# Setting for the new UTF-8 terminal support in Lion
export LC_CTYPE=en_US.UTF-8
export LC_ALL=en_US.UTF-8

If you are using Z shell (zsh), edit file zshrc:

# Setting for the new UTF-8 terminal support in Lion
LC_CTYPE=en_US.UTF-8
LC_ALL=en_US.UTF-8