my $skip = 0;
while ($skip < length($str)) {
    my ($f1_len, $f2_len, $f3_len) = unpack("x${skip}aaa", $str);
    $skip += 3;
    my $field1 = unpack("x${skip}a$f1_len", $str);
    $skip += $f1_len;
    # etc
}