my @merges;
    my $twig = ...
        'Rs/MergeHistory' => sub {
             my $rsid = ...;
             my $buid = ...;
             push @merges, [ $rsid, $buid ];
        },
    ...
    print $out join(',', map { $_->[0] } @merges ),
               "\t",join(',',map { $_->[1] } @merges ), "\n";