#!/usr/bin/perl -w

use strict;
use Data::Dumper;

my %result;

for my $l (<DATA>) {
    my ($cod,$desc) = ($l =~/(\d*) (.*)/);
    my $hashref = \%result;
    my $partial = '';
    foreach my $layer (split //, $cod) {
        $partial .= $layer;
        $hashref = ($hashref->{$partial} ||= {});
    }
    $hashref->{0} = $desc;
}
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