my $r = Apache2::Request->new();

    while($file_is_not_finished) {
         ... write a bunch of stuff to $file
         $r->print(" ");
         $r->rflush();
    }

    $r->content_type('application/zip');
    $r->headers_out->set(
        'Content-disposition' => "attachment;filename=\"file.zip\""
    );

    my $fh;
    open($fh, $file) or die $!;
    $r->print(<$fh>);