The error occurs when you assign to a negative index so large that it indicates a negative element.

$ perl -e'   @a = qw( abc def );
             $a[-1] = "ghi";
             $a[-2] = "jkl";
             $a[-3] = "mno";       '
Modification of non-creatable array value attempted, subscript -3 at -
+e line 4.
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In your code, that will happen when an empty array is passed to shuffle. Fix by changing

while ( --$i )
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to

while ( --$i > 0 )
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