while( read DATA, $data, BLOCKLEN ) {
    ## Extract the header from the front of the block
    my $header = substr $data, 0, BLOCKHDR, ''; 
    
    ## do stuff with $header
    
    for my $cdr ( 1 .. MAXRECORD ) {
        ## Extract each record in turn from $data
        my $record = substr $data, 0, CDRLEN, '';
        
        ## do stuff with $record
    }
    my $trailer = $data; ## Anything left in $data should be your trailer.
    
    ## do something with it.
}