$ perl -Mstrict -Mwarnings -E '
    my @list = qw{12 123 1234 12345 123456 1234567 234 2345 23456};

    for (@list) {
        s/\w{3}\K/-/g;
        say;
    }
'
12
123-
123-4
123-45
123-456-
123-456-7
234-
234-5
234-56