my $obj = bless {
	Root => $rootDir, 
	UserDir => $obj->{Root} . "Usr/$userId/",
	UserAppData = $obj->{UserDir} . "$appName/",
}, shift; #  of cause this won't work

##</code><code>##

$obj = bless {
	Root => $rootDir, 
	UserDir => undef,
	UserAppData => undef,
}, shift;

$obj->{UserDir} = $obj->{Root} . "Usr/$userId";
$obj->{UserAppData} = $obj->{UserDir} . "$appName";

##</code><code>##

[ConfigDirs]
  A = /some/dir
  B = /_#A#_/DeeperDir
  C = /_#B#_/EvenDeeperDir

##</code><code>##

$var = {
  'ConfigDirs' => {
    'A'=> '/some/dir',
    'B'=> '/some/dir/DeeperDir',
    'C'=> '/some/dir/DeeperDir/EvenDeeperDir'
  }
}