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

my %hash;
while (my $line = <DATA>) {
    chomp $line;
    my ($number, $description) = split /\s/, $line, 2;
    push @{$hash{$description}}, $number;
}
print Dumper \%hash;

__DATA__
00001 Description1
00002 Description2
00003 Description1