G'day hankcoder,

Welcome to the monastery.

"s/(\w{3})\B/$1-/g" does what you want:

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

    for (@list) {
        s/(\w{3})\B/$1-/g;
        say;
    }
'
12
123
123-4
123-45
123-456
123-456-7
234
234-5
234-56
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