> I originally tried to have two XML documents open and copy/paste items from one into the other

Well, possible it's possible, not beauty (and I added some spaces..;) but..

perl -MXML::Twig -e "BEGIN{$f=shift @ARGV}

map{ XML::Twig->new(twig_handlers => { 'newitems/book' => sub { push @
+content, $_; $_->cut }})->parsefile($_) }@ARGV;   

XML::Twig->new(pretty_print=>'indented',twig_handlers => {'catalog'
=> sub {foreach my $content ( @content ) {$content->paste( last_child 
+=> $_ );}}})->parsefile($f)->flush

" current_catalog.xml new_items.xml
[download]

L*

PS updated, obfuscated and shortened

perl -MXML::Twig -e "map{$t=XML::Twig->new(twig_handlers=>{$s?('catalo
+g'=>sub{foreach$c(@c){$c->paste(last_child=>$_)}}):('newitems/book'=>
+sub{push@c,$_;$_->cut})})->parsefile($_);$s++;}@ARGV;$t->flush"  newb
+ook.xml catalog.xml
[download]