I believe this returns the structure you requested:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $tree ;
my %tree_levels;

while ( <DATA> ) {
  chomp;
  m/^(\s+)?/;
  my $length;
  if ( defined $1 ) {$length = length($1);} 
  else { $length = 0 }

  my $hr = { label => $_, children => [] };
  
  $tree_levels{$length} = $hr;
  
  unless ( defined $tree ) { $tree = $hr; }  
  else {
    push @{$tree_levels{$length - 2}->{children}}, $hr;
  }
}

print Dumper $tree;

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

-enlil