Hey, sorry for the slow response. I left for a vacation shortly after I posted my question. The filehandle is not in scope because the handlers are defined in a module and passed to the twig like this: (paraphrased)

...
use Handlers::Library qw/:handlers/;
...
open my $filehandle, '>', 'output.xml' or die ">output.xml:$!";
my $p = XML::Twig->new(
    twig_handlers => { foo => \&foo_handler },
);
[download]

I'm going to end up doing what I was trying to avoid in the first place: passing a filehandle to all of the ->flush calls. When I asked the original question, I figured this was something XML::Twig would naturally support. (It's wonderfully DWIMmy, and does something similar for defaulting a filehandle with twig_print_outside_roots => $filehandle, but I didn't realize that that was just a select.)

So, long story short, my final code will be something like: (untested)

package Handlers::Library;
my $twig_fh;
BEGIN { $twig_fh = *STDOUT; }
sub twig_output {
    my $name = shift;
    open $twig_fh, '>', $name or die ">$name: $!";
}
sub foo_handler { ... $twig->flush($twig_fh) }
sub bar_handler { ... $twig->flush($twig_fh) }
[download]

use Handlers::Library qw/:handlers/;
twig_output('output.xml');
my $p = XML::Twig->new(...);
[download]