Unable to set custom header

Hello,

I am trying to set a custom header through a mod_perl handler.

My perl handler code is:

sub handler {
    my $r = shift;

    $r->headers_out->add("foo" => "bar");
    $r->log->error($r->connection->remote_ip()." is client IP");

    return Apache2::Const::OK;
}

The handler gets called as I am able to see the log message, but the header
is not emitted.

My Apache configuration is -

  ProxyRequests On
  ProxyVia On
  ProxyRemote * http://localhost:3128

  SetHandler modperl
  PerlPostReadRequestHandler +Foo::Proxy

  <Location />
            ProxyPass http://www.something.com/
            ProxyPassReverse http://www.something.com/
   </Location>

When I do the same through mod_header like this it works -

RequestHeader set foo bar

I want foo: bar to go as a header in the HTTP request to my proxy on port
3128.

Can anyone advise why my perl handler doesn't work when it should be doing
the same thing as mod_headers?

Regards,
Ashish