Need help with Apache2::Subprocess undefined value error when using spawn_proc_prog

[aels]

Hi,
I need to test the use of Apache2::SubProcess to execute an external command from a web page.
To create a small test case I use the following:

#!/usr/bin/perl -w
use strict;
use CGI qw/:standard/;
use Apache2::SubProcess ();

my $out;
my $r = shift;
my $command = "/bin/echo 123 > /tmp/test";

    print header;
    print "<p>";
    print "test";
    print "</p>";

    $out = $r->spawn_proc_prog($command);

    print end_html;

But when I execute this, I get:
Can't call method "spawn_proc_prog" on an undefined value at /srv/www/cgi-bin/pstest.pl line 15

Any ideas what I am doing wrong.
Thank you in advance.