Further to Eily's post: Instead of
    print $$self $data;
try
    print { $$self } $data;
(untested).

Yes, the block returning the dereferenced file handle is doing the job.

Even if it is not very explicit in the documentation, it is suggested in the last paragraph.

It does seem so, but I couldn't find that piece of code in XML::SAX::Writer, which is not really surprising since this is a syntax error, not just a warning.

Well, I've just checked and it is not the actual package code.
I remember now that I found the example of custom ConsumerInterface in the documentation of XML::SAX::Writer

    sub output {
        my $self = shift;
        $$self .= uc shift;
    }
[download]

and I have adapted it to my needs: replacing the string scalar to store the data by a file handle. Thus I am responsible for print $$self $data;. Actually I was surprised for not having the warning while using the stock module... Mea culpa.

What line 3 ($$self .= uc shift) is saying (just for those not understanding the above code) is that it dereferences the thingy that $self points to, and appends the upper cased version of the next parameter to thingy, putting the results back into thingy.

--MidLifeXis