use charnames qw();

my $s = "Müller";
my $t = "";
for my $ch (split //, $s) {
    my $ord = ord $ch;
    if ($ord > 127) {
        my $charname = charnames::viacode($ord);
        if ($ord == 223) { # Ausnahme sz
            $t .= "ss";
        } elsif ($charname =~ /(CAPITAL|SMALL)\s+LETTER\s+(.)/) {
            if ($1 eq 'CAPITAL') {
                $t .= uc($2);
            } else {
                $t .= lc($2);
            }
        } else {
            $t .= "?";
        }
    } else {
        $t .= $ch;
    }
}
warn $t;