Q:how to get the byte amounts

[Lijf]

I want to get the byte amounts that sent to the client via Apache, so I write the 
following code

#!/usr/bin/perl
use strict;
package Apache::GetByte;
use Apache::Constants qw(:response);
use Apache ();
sub handler{
    my $r = shift;
    my $byte=$r->bytes_sent;
    warn "BYTE = $byte \n";
    return DECLINED;
 }
1;

In apache httpd.conf 
<Directory "public_html">
  SetHandler perl-script
  PerlLogHandler Apache::GetByte
</Directory>


but the result is BYTE = 0 in apache error_log. Why? What shall I do? 


Yours sincerely

Li junfeng
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