reference to an undefined key

[exilepanda]

Hello monks,

Sorry I believe I have given a pretty poor title but I really can't think of any better one, and I think this is the fact though. I'd like to know if there's any chance I can have such code candies ?

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

instead of :

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

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

In fact I have much more paths that I need to defined while the object is created, but I hope to state the logic of the properties in clear at the first place.

I guess I am out of luck but still trying to ask because when the interpreter read {Root}, that should be some place already stored the value ( but I juz don't know how to ref to it), no?

UPDATE:

I know why it's not working, what I try to emphasize is only inside the hash, so please forgive I didn't manage to give good code about the before and after.

Though, I suddenly recalled something ( thank you very much for all you guys' feedback ). Yrs ago, I wrote a module for interpret an INI config file. Which accepts things like this:

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

And my module will go through it line by line. Every key recorded can be used by the next line. Regex will replace the _#*#_ with a proper recorded key's value. So it Data::Dump will produce something like

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

Now the questions can be added are: 1) is there any existing module deal's something like that? 2) is there any threat I elaborating things in this way?

As I am going to reuse this module's code to let me accept params as I originally proposed.