use warnings;
use strict;
my %data;
while (<DATA>) {
    chomp;
    next unless /\S/;
    my ($k,$v) = /^\s*(\S+)\s+(.+?)\s*$/;
    push @{$data{$k}}, $v;
}
use Data::Dumper;
print Dumper(\%data);
__DATA__
1 AAA 
2 BBB
3 CCC
4 DDD
2 EEE

##</code><code>##

$VAR1 = {
          '3' => [
                   'CCC'
                 ],
          '1' => [
                   'AAA'
                 ],
          '4' => [
                   'DDD'
                 ],
          '2' => [
                   'BBB',
                   'EEE'
                 ]
        };