Now maybe is the time to investigate the difference between the two values.

Have you printed out their length? Have you compared their values as Perl perceives them?

...
my $last_page;

foreach my $page (@files) {
    warn "Now opening different page.";
    warn "Old: [$last_page] New: [$page]";
    warn sprintf "Length: Old: %d New: %d", length $last_page, length 
+$page;
    warn "They are " . ($last_page ne $page ? "not" : "" ) . " equal";

    my $xp = XML::XPath->new(filename => $page);
    $last_page= $page;
...
};
[download]