I'm working on a set of modules that read and write binary files. It's quite long, but here are the interesting parts :
First, I store the data in a hash: 
my $_validfields = {} ;
# afin que l'ordre des champs soit conservé
tie %{$_validfields}, "Tie::IxHash";

%$_validfields = (
 'LENGTH' => {
     offset    => 0,
     length    => 5,
     format    => 'A',
     default      => 175,
     mandatory => 1
 },
 'RECORD FORMAT' => {
     offset    => 5,
     length    => 1,
     format    => 'A',
     default      => ' ',
     mandatory => 1
 },
 'ID' => {
     offset    => 6,
     length    => 20,
     format    => 'A',
     default      => 'GEST',
     mandatory => 1
 },
 'MEDIA' => {
     offset    => 26,
     length    => 2,
     format    => 'A',
     default      => 20,
     mandatory => 1
 },
 'ORIG' => {
     offset    => 28,
     length    => 5,
     format    => 'A',
     default      => 'GEST',
     mandatory => 1
 },
 'STATE' => {
     offset    => 33,
     length    => 1,
     format    => 'A',
     default      => 0,
     mandatory => 1
 },
 'STATUS' => {
     offset    => 34,
     length    => 5,
     format    => 'A',
      default      => '',
    mandatory => 1
 },
 'ERROR RECORD' => {
     offset => 39,
     length => 5,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'PRIORITY' => {
     offset => 44,
     length => 1,
     format    => 'A',
      default      => 9,
     mandatory => 1
 },
 'TARGET SERVER' => {
     offset => 45,
     length => 4,
     format    => 'A',
      default      => '',
    mandatory => 1
 },
 'TARGET LINE' => {
     offset    => 49,
     length    => 2,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'OUTPUT SC' => {
     offset    => 51,
     length    => 4,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'OUTPUT LINE' => {
     offset    => 55,
     length    => 2,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'BUNDLE FNAME' => {
     offset    => 57,
     length    => 47,
     format    => 'hA',
      default      => 0x01,
     mandatory => 1
 },
 'DUPS' => {
     offset    => 104,
     length    => 5,
     format    => 'A',
      default      => 1,
     mandatory => 1
 },
 'ORDER INITIATE TS' => {
     offset => 109,
     length => 14,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'ORDER CREATE TS' => {
     offset    => 123,
     length    => 14,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'ORDER READ TS' => {
     offset    => 137,
     length    => 14,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'ORDER CC TS' => {
     offset => 151,
     format    => 'A',
      default      => '',
     length => 14
 },
 'COPIES REQUIRED' => {
     offset    => 165,
     length    => 5,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 'DISKS PRODUCED' => {
     offset    => 170,
     length    => 5,
     format    => 'A',
      default      => '',
     mandatory => 1
 },
 # params optionnels
 #****v*
 # COLLATE est le seul paramètre codé par sa valeur binaire
 # c'est donc un INT et pas un STRING
 # ( #!%? et le coeur y est)
 #***
 'COLLATE' => {
     offset    => 175,
     length    => 1,
     format    => 'C',
      default      => 0,
     mandatory => 0
 },
 'RETURN INFO' => {
     offset    => 177,
     length    => 1,
     format    => 'A',
      default      => '',
     mandatory => 0
 },
 'RETURN FILE' => {
     offset    => 178,
     length    => 260,
     format    => 'A',
      default      => '',
     mandatory => 0
 },
 'EXT BUNDLE FNAME' => {
     offset    => 438,
     length    => 260,
     format    => 'A',
      default      => '',
     mandatory => 0
 },
 'EMAIL' => {
     offset    => 698,
     length    => 128,
     format    => 'A',
      default      => '',
     mandatory => 0
 },
 'INPUT BIN' => {
     offset    => 826,
     length    => 2,
     format    => 'A',
      default      => '',
     mandatory => 0
 },
);

[download]
Then there are a couple of subs to manage this, here is the important part for writing : 
sub validfields {
    return $_validfields
}

sub _packdata {
    #****m* DataFile/_packdata
    # USAGE
    # $file->_packdata()
    # sérialise pour pack() tous les records
    # d'un fichier
    #***
    my $self = shift ; 
    
    my @stringrecords ;
    
    foreach my $record_ref ( @{ $self->{records} } ) {

        # il faut déréférencer l'objet record avant de l'appeler
        my $recdata = $self->_serialize_fields( $$record_ref->getdata(
+) );
        push @stringrecords, $recdata 
            if $recdata ;
    }
    
    @stringrecords ? return join ( '', @stringrecords ) : return
    
}

sub _serialize_fields {
    #****m* DataFile/serialize_fields
    # USAGE
    # $file->_serialize_fields( $data_href )
    # sérialise les données d'un record
    #***
    my $self = shift ;
    my $data = shift ;
    
    my $output = '';
    
    my $allfields = $self->validfields() ;
    
    # on itère sur tous les champs possibles
    foreach my $k ( keys %$allfields ) {
        # on insère la longueur en dernier
        next if $k eq 'LENGTH' ;
        
        my $value ; 
        ( exists $data->{$k} ) ? ( $value = $data->{$k} ) : ( $value =
+  $allfields->{$k}{default} ) ;
        
        # le champ COLLATE est le seul qui soit un BYTE
        # les champs "noms courts" doivent contenir 0x01
        # la longueur est corrigée de la longueur du format
        $output .= pack( $allfields->{$k}{format}. ( $allfields->{$k}{
+length} - length($allfields->{$k}{format}) + 1 ), $value ); 
        
    }
    # on insère la longueur du champ au début
    my $datalength = length($output) + 5;
    $output = pack( "A5",$datalength ) . $output ;

    print $output ;
    return $output
}

[download]
Hope this helps.

In reply to Re: Recreating a binary file from hex representation by wazoox
in thread Recreating a binary file from hex representation by choedebeck

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