#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $info = {
  '12' => '11',
  '11' => '10',
  '10' => '888',
  '17' => '42',
  '999 ' => '42',
  '42' => '4711',
  '23' => '4711',
};

my %reverse;

for my $key ( keys %{$info} ) {
    my $parent = $info->{$key};
    push @{ $reverse{$parent} }, $key;
}

my @parents = values %{$info};
my @roots   = grep{ !exists $info->{$_} }@parents;

my %tree;
for my $root ( @roots ) {
    fill_tree( \%tree, \%reverse, $root );
}

print Dumper \%tree;

sub fill_tree {
    my ($subtree,$reverse,$root) = @_;
    
    if ( exists $reverse->{$root} ) {
        $subtree->{$root} = {};
        
        for my $node ( @{ $reverse->{$root} } ) {
            fill_tree( $subtree->{$root}, $reverse, $node );
        }
    }
    else {
        $subtree->{$root} = -1;
    }
}