I would change that test to:

if ($attrib & 16) { return 1 }

What you're doing is testing bit 4 of the file attributes, which is probably the "directory" bit. But if this was a read-only, hidden, etc directory, that $attrib would be something else, like 17.

If that doesn't make sense, here's a hint; ask if this isn't too clear:

         You want to test this bit
        v
16 = 00010000b
17 = 00010001b
        ^
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