What I want is to print each MergeHistory rsId and buildId together as the following:

What has to happen for you to be able to do that?

You have to keep a stack of rsids and buildids

so

    my @rsids;
    my @buildids;
    my $twig = ...
        'Rs/MergeHistory' => sub {
             my $rsid = ...;
             my $buid = ...;
             push @rsids, $rsid;
             push @buildids, $buid;
        },
    ...
    print $out join(',',@rsids),
               "\t",join(',',@buildids), "\n";
[download]

or

    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";
[download]