I figured out what I was doing wrong, so I thought I'd document it, because it took me so long to figure out.

According to http://httpd.apache.org/docs/2.2/mod/core.html#namevirtualhost, "The NameVirtualHost directive is a required directive if you want to configure name-based virtual hosts."

I am specifying IP-based virtual hosts and so thought that a NameVirtualHost directive was not required. But it didn't work.

When I added the NameVirtualHost directive, it all worked swimmingly.

So, to dynamically configure virtual hosts, you can do as follows:

In httpd.conf:
----------------------
  PerlConfigRequire "conf/startup.pl"
----------------------

In conf/startup.pl:
----------------------
  use Apache2::ServerUtil();

  my $config = '';
  foreach my $site (@sites) {
    $config.= = <<CONFIG;
        NameVirtualHost $site->{ip}:80
        <VirtualHost $site->{ip}:80>
            ServerName $site->{name}
            # Other config details
        </VirtualHost>
  CONFIG

  my $server = Apache2::ServerUtil->server;
  $server->add_config([split /\n/, config]);
----------------------
[download]

hope this saves somebody else some pain