Well, if you don't count $_ as an extra variable because it's already implicitly there, you can do it like this:

    use bytes;
    if (vec($s,3,2) < 3) {
        vec($s,$_,8) = vec($s,$_+1,8) for 0..length($s)-2;
        chop($s);
    }
    else {
        vec($s,$_,8) = vec($s,$_+1,8) for 0..length($s)-2;
        chop($s);
        while (vec($s,3,2) == 2) {
            vec($s,$_,8) = vec($s,$_+1,8) for 0..length($s)-2;
            chop($s);
        }
    }
[download]

Update:
Hmm, here's a solution without $_:

    use bytes;
    if (vec($s,3,2) < 3) {
        () = $s =~ m/.(?=.)(?{ vec($s,pos($s)-1,8) = vec($s,pos($s),8)
+})/sg;
        chop($s);
    }
    else {
        () = $s =~ m/.(?=.)(?{ vec($s,pos($s)-1,8) = vec($s,pos($s),8)
+})/sg;
        chop($s);
        while (vec($s,3,2) == 2) {
            () = $s =~ m/.(?=.)(?{ vec($s,pos($s)-1,8) = vec($s,pos($s
+),8)})/sg;
            chop($s);
        }
    }
[download]