I know I'm a little late, but I like these little 'perl-doodles'. Here's how I'd do it.

use strict;
use warnings;

my %hash;
while ( <DATA> ) {
    chomp;
    my ( $key, $value ) = split /\s+/, $_, 2;
    push @{$hash{$key}}, $value;
}

print "$_: @{$hash{$_}}\n" foreach keys %hash;

__DATA__
243_405 35 23 13
243_405 46 21 15
241_333 65 32 20
241_333 52 44 11
[download]

...tested, of course.