# ...

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