It seems to me that your data structure is a little clumsy for what you are trying to do. You are maintaining two separate arrays of file names. If I were to do this, I would probably use a data structure something like:

$VAR1 = [
          {
            'filenames' => [
                { large => 'fIyvYwJWmstR.jpg', small => 'lAwKmMFJVQDI.
+jpg', }
                ...
            ],
            'pos' => 1,
            'title' => 'February event (Feb 2013)',
            'link' => 'february-event-feb-2013',
            'bookid' => 8,
          },
[download]

HTH.