Debian Locale Configuration - Shammer's Philosophy

I have tried to configure locale correctly before like How to fix set locale error? - Shammerism, but I was faced with the case that the way written in this article doesn't recover the locale. After editing /etc/environment, the following error still appears with perl -v.

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

The result of locale -a in this environment is below.

# locale -a
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_COLLATE to default locale: No such file or directory
C
C.UTF-8
POSIX

Not installed en locale. This is a test environment so I don't want to add new locale. I should edit /etc/environment like below.

# cat /etc/environment 
LANG=C
LC_ALL=C
LC_CTYPE=C.UTF-8