This is some of my coding..could you comment ?. I read the record length as $length = unpack "C4", substr $data,0,2,'';. is it correct ?

read (DATA, $data, 3);
$tag    = unpack "H2", substr $data,0,1,'';

if ($tag =~ /(81|80|00)/) {
    # the Record Identifier H’80 is used as Filler record to fill in o
+ne byte
    if ($tag == "80") {
        $length = 2;
    }

    # the Record Identifier H’81 is used as Filler record to fill in s
+everal bytes
    if ($tag == "81") {
        $length = unpack "C4", substr $data,0,2,'';
        $length -= 3;
    }

    # the Record Identifier H’00 is used as Filler record to fill in 3
+2 bytes
    if ($tag == "00") {
        $length=62;
    }

    read (DATA, $data, $length);
    $raw = unpack "H*", $data;

    if ($trace) {
        print  ("FILLER\n");
        printf ("%-20s : %-5s\n", "TAG", $tag);
        printf ("%-20s : %-5s\n", "RECORD LENGTH", $length);
        print  ("RAW DATA : \n$raw\n");
    }
}
[download]