#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
$Data::Dumper::Indent = 1;

my $tree;
my @refs;

while (<DATA>) {
  chomp;

  my ($prefix) = /^(\s+)/;
  s/^$prefix// if $prefix;
  my $lvl = $prefix ? (length $prefix) / 2 : 0;

  my $rec = { label => $_, children => [] };
  $refs[$lvl] = $rec;
  $#refs = $lvl;

  if ($tree) {
    push @{$refs[$lvl - 1]->{children}}, $rec;
  } else {
    $tree = $rec;
  }
}

print Dumper $tree;

__DATA__
Building
  House
    Window
      Glas
        Silicium
    Door
    Roof
      Wood
  Hut
    Pizza
  Garage
    Door