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.
}

##</code><code>##

until( eof DATA ) {
    read DATA, my $header, BLOCKHDR or die;
    ## Do stuff with $header;
    
    for my $cdr ( 1 .. MAXRECORD ) {
        read DATA, my $record, CDRLEN or die;
        ## Do stuff with $record
    }
    
    read DATA, my $trailer, BLOCKTRL or die;
    ## Do stuff with $trailer
}