Just what is unclear? There's not much to explain I think, so I'm not sure what to clarify. What you'd do is basically something like this:

# ...

my @valid_page = qw(order cart shelf intro register);
my %collected_data = (foo => 1, bar => 2, );

my $requested_page = $cgi->param('page');
my ($actual_page) = grep $requested_page eq $_, @valid_page;

print("Content-type: text/plain\n\nNo valid page requested."), exit
    unless $actual_page;

my $container = HTML::Template->new_file('container.tmpl');
my $template = HTML::Template->new_file($actual_page . '.tmpl');

$template->param(%collected_data); # fill in actual page

$container->param(
    %collected_data,
    CONTENT => $template->output, # insert into container
);

print "Content-type: text/html\n\n", $container->output; # we're done
[download]

You would then have order.tmpl, cart.tmpl, shelf.tmpl, intro.tmpl, register.tmpl and container.tmpl in a directory, and the latter would have a <TMPL_VAR NAME="CONTENT"> somewhere, so that the other template's output is inserted at that point. If you need any pointers, tell me what you have trouble with.

Makeshifts last the longest.