#!/usr/bin/perl --

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

my %result = ();

foreach my $line (<DATA>) {
    chomp $line;
    my ($index, $value) = split(/ /, $line, 2);
    my $temp = \%result;
    foreach my $path (split (//, $index)) {
        $temp = $temp->{$path} ||= {}
    }
    $temp->{0} = $value;
}

print Dumper \%result;

__DATA__
1 ITEM
11 Sub Item 1
111 Item X
112 Item
1121 Another Item
11212 And Another Item
12 Sub Item 2

##</code><code>##

my %result = ();

foreach my $line (<DATA>) {
    chomp $line;
    my ($index, $value) = split(/ /, $line, 2);
    my $temp = \%result;
    foreach my $path (split (//, $index)) {
        $temp = $temp->{$path} ||= {}
    }
    $temp->{''} = [$index, $value];
}