There would be two ways for you to go :

• Use an external file for your banner ad.
  This will be the easiest method to add to your code, as you simply check if there is a file (called, say, banner.ad), read it in and print it to the user. The problem with this approach is that webserver permissions and the concept of current directory are obstacles to be circumvented. Here is some (untested) example code that should get you started in this direction :

  function getAd {
    my ($filename) = @_;
    my $result = "<!-- No HTML ad -->; # the HTML
  
    use FindBin;
    $filename = $FindBin::Bin . "/" . $filename;
  
    if ( -f $filename )  {
      { 
        local $/;
        open F, "< $filename" or die "Couldn't open '$filename' : $!";
        $result = <F>;
        close F;
    };  
    return $result;
  };
  [download]

• Use Here-Documents for your banner ad.
  This is not so easy to set up, as you will have to modify your CGI every time the ad changes, but if you got your CGI running, you also get the ad running. A here-doc means that Perl will read all the text from <<'MARKER' until it encounters the end-marker MARKER at the start of a line all by itself. Some code :

  function getAd {
    return <<'END_OF_AD'
  <a href="http://www.bar.com/gotcha"><img src="http://www.bar.com/foo.g
  +if" alt="Foo">
  </a>
  END_OF_AD
  };
  [download]

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
[download]