You didn't use the same pack() pattern that perldesignpatterns.com does. That page shows pack('LL', 15, 0) while your original example uses 'll'. Uppercase 'LL' and lowercase 'll' are a bit different.
Perhaps more importantly, that page explains why it uses 'LL' and how to determine what should work for you. The instructions look good, so I recommend following them (if you haven't already).
You may wish to carefully compare the example with your code. You may be overlooking some other difference.
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