I haven't tested this either, but what about this:

sub ReadSource{
    my $DataSource =    shift;
    my $HashRef    =    shift;
    
    foreach my $ItemID (@Data) {
        defined $HashRef->{$ItemID}
        ? push @{$HashRef->{$ItemID}},  $DataSource
        :        $HashRef->{$ItemID} = [$DataSource];
    }
}
[download]

cLive ;-)

edit - nevermind, I forgot about autovivifying. It's late dammit...