I can't get shmem's solution to work unless I add use utf8 and operate on LC_CTYPE. Based on the Location, one might guess that shmem's LC_CTYPE is already 'de_DE'?

$ perl -l
use strict;
use warnings;
use locale;
use POSIX qw(locale_h);
print setlocale(LC_COLLATE,"de_DE") or die "foo? - $!\n";
("fähler"=~/^\w+$/) ? print "yup" : print "nope";
__END__
de_DE
nope
[download]

$ perl -l
use strict;
use warnings;
use utf8;
use locale;
use POSIX qw(locale_h);
print setlocale(LC_COLLATE,"de_DE") or die "foo? - $!\n";
("fähler"=~/^\w+$/) ? print "yup" : print "nope";
__END__
de_DE
nope
[download]

$ perl -l
use strict;
use warnings;
use utf8;
use locale;
use POSIX qw(locale_h);
print setlocale(LC_CTYPE,"de_DE") or die "foo? - $!\n";
("fähler"=~/^\w+$/) ? print "yup" : print "nope";
__END__
de_DE
yup
[download]