my $ua = LWP::UserAgent->new( agent => $useragent );
open my $fh, '>:raw', $file or die $!;
my $res = $ua->get( 
    $url, 
    ':content_cb' => _return_callback( $fh ),
);
my $str = $res->request->as_string;
say $str;

# GET http://hostname.com/videoplayback?...
# User-Agent: Mozilla/5.0