open my $FH, '>', "file_difference_report" or die $!;
my @base_file_names = ( 'file1', 'file2', 'file3', 'file4' );
for my $file_name (@base_file_names) {
   if (! -e "$file_name.xml") {
      print "$file_name.xml: Not present ... not interesting file?\n";
      next;
    }
    if (! -e "$file_name.bak") {
        print "$file_name: no backup, so probably not changed\n";
        next;
    }
    # If we get here, we have a .bak and a .xml file, so make another program
    # compare them for us:
    my $output = `diff $file_name.xml $file_name.bak`;
    print $FH "\n\n===== $file_name changes =====\n";
    print $FH $output;
    print $FH "\n\n";
}