#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my @files = @ARGV[0, 1];

my %extracted;

for my $xml_file (@files) {
    my $dom = 'XML::LibXML'->load_xml(location => $xml_file);
    for my $file ($dom->findnodes('/root/file')) {
        my $original = $file->{original};
        for my $unit ($file->findnodes('body/unit')) {
            my $id = $unit->{id};
            my $title = $unit->findvalue('title');
            $extracted{$original}{$id}{$xml_file} = $title;
        }
    }
}

for my $file (keys %extracted) {
    for my $id (keys %{ $extracted{$file} }) {
        say join "\t", $file, $id, @{ $extracted{$file}{$id} }{@files};
    }
}