'/shipment/box' => sub { _move(@_); 1; }

##</code><code>##

# '/shipment/box' => sub { _move(@_); 1; } #wrong
# to
'/shipment/box' => \&_move,

##</code><code>##

sub _move {
    unless ($_[1]->descendants('slip')){
         $_[0]->purge();
         return;
    }
    foreach my $descendant ...


##</code><code>##

#!/usr/bin/perl
# use 5.020; unneeded aside from the say
use strict;
use warnings;
use XML::Twig;

##
## This works as expected
##
my $working_xml = XML::Twig->new(
    twig_handlers => {
        #'/shipment/box' => sub { _move(@_); 1; },  #why? and why 1; at the end?
        '/shipment/box' => \&_move,
    },
    pretty_print => 'indented',
)->safe_parse(
<<"XML"
<?xml version="1.0" encoding="UTF-8"?>
<shipment>
    <box>
        <packing>
            <slip>potato</slip>
            <slip>pear</slip>
            <slip>peach</slip>
        </packing>
    </box>
    <box>
        <packing>
            <slip>apple</slip>
        </packing>
    </box>
</shipment>
XML
);

print "\n\n---------------------\n\n";


##
## This ALSO WORKS FINE NOW!!
##
my $broken_xml = XML::Twig->new(
    twig_handlers => {
        '/shipment/box' => \&_move,
    },
    pretty_print => 'indented',
)->safe_parse(
<<"XML"
<?xml version="1.0" encoding="UTF-8"?>
<shipment>
    <box>
        <packing>
            <slip>potato</slip>
            <slip>pear</slip>
            <slip>peach</slip>
        </packing>
    </box>
    <box>
        <packing>
            <note>nothing here!</note>
        </packing>
    </box>
</shipment>
XML
);

# ----------

sub _move {
    unless ($_[1]->descendants('slip')){
         $_[0]->purge();
         return;
    }
    foreach my $descendant ( $_[1]->descendants('slip') ) {
        $descendant->set_att('_TYPE' => 'produce');
        $descendant->move('before', $_[1]);
    }
    $_[1]->delete;
    $_[0]->flush();

    #1;  # why?
}


## OUTPUT


<?xml version="1.0" encoding="UTF-8"?>
<shipment>
  <slip _TYPE="produce">potato</slip>
  <slip _TYPE="produce">pear</slip>
  <slip _TYPE="produce">peach</slip>
  <slip _TYPE="produce">apple</slip>
</shipment>


---------------------

<?xml version="1.0" encoding="UTF-8"?>
<shipment>
  <slip _TYPE="produce">potato</slip>
  <slip _TYPE="produce">pear</slip>
  <slip _TYPE="produce">peach</slip>
</shipment>