my $factory = Config::Builder->new qw(
  user => user
  admin => admin
  htmldir => %path $basedir html
  userdir => %path $htmldir $user
  admindir => %path $htmldir $admin
  email => %join $username @ $domain
);

my $values = read_your_config_file(); # returns a hash of values
my $config = $factory->make_config($values);

print "user dir is ".$config->("userdir")."\n";
print "admin dir is ".$config->("userdir")."\n";

##</code><code>##

$values = {
	basedir => "/base/dir"
}

/base/dir/html/user
/base/dir/html/admin
----

##</code><code>##

$values = {
	basedir => "/base/dir",
	user => "customuser",
}

/base/dir/html/customuser
/base/dir/html/admin
---

##</code><code>##

$values = {
	basedir => "/base/dir",
	htmldir => "/custom/html/dir",
}

/custom/html/dir/user
/custom/html/dir/admin

##</code><code>##

my $c = read_your_config_file(); # returns a hash of values
use File::Spec::Functions;
$c->{user} ||= "user";
$c->{admin} || = "admin";
$c->{htmldir} ||= catfile($c->{basedir}, $c->{html});
$c->{userdir} ||= catfile($c->{htmldir}, $c->{user});
$c->{admindir} ||= catfile($c->{htmldir}, $c->{admin});
$c->{email} ||= "$c->{username}\@$c->{domain}";