Here is pseudocode.

The record fields can be either fixed or array. Fixed data type: Character, Integeral types (Unsigned short, Unsigned long), Float Variable data type: String(pascal style C/a), Array of Unsigned short, etc Optional fields at end of record can be omitted. So if there is a record with optional (Byte, C/a(String), Float), that needs to be handled somehow.

read_file_header
determine endian from file_header data
set up unpack data types(for big/little endian)
if(endian eq 'little') {
   $REAL_TEMPLATE = "R<";
   $U2 = "n";
   $REC_HEAD = ...
} else {
   $U2 = "v";
   $REC_HEAD = ...
   ...etc
}
while(1) {
    read (REC_HEAD) size data from file
    ($rec_len,$rec_type,....) = unpack($REC_HEAD)
    my $rec_body = read($rec_len)
    # a big switch on rec_type
    if($rec_type == FOO) {
       # unpack record_body somehow for THIS rec type
       # FOO can be  
       # below 4 fields must be present in record body
       # fixed uSHORT,uLONG,uLONG,string(C/a), 
       # below are optional
       # optional Byte(Optional onwards this field), Float,String
       my @data = unpack(" $uSHORT $uLONG $uLONG C/a",$rec_body);
       my $consumed_length = 10 + length($data[-1]) + 1;  # ushort +2*
+ulong + length(C/a)
       if($consumed_length < $rec_len) {
          # optional fields present
          push @data, unpack("x${consumed_length} C",$rec_body);
          $consumed_length += 1;
       }
       if($consumed_length < $rec_len) {
           push @data,unpack("x${consumed_length} $Float",$data);
           $consumed_length += 4;   # float is 4 bytes
       }
       # next optional ..etc
    }
    elsif($rec_type == BAR) {

    }
    
}
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