$ export LANG="en_US.UTF-8" ; perl -E "say 'testärger' ~~ /^\p{Alpha}+$/i"
1

$ export LANG="en_EN.UTF-8" ; perl -E "say 'testärger' ~~ /^\p{Alpha}+$/i"
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = (unset),
        LANG = "en_EN.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
1