This is my attempt at decoding what's going on: 
1) @foo  = 
    the result of dereferencing $_[0]

2) @bar = 
    the result of dereferencing $_[0]
        if $_[0] is true
    otherwise the result of dereferencing the empty array "()"

3) @jazz = 
    the number of elements of the result of dereferencing $_[0]
        if that number is true (greater than 0)
    otherwise the empty array "()"

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So, depending on your arguments, both 2 and 3 may give unexpected results, and all three may cause problems if $_[0] is not an array reference

In reply to Re: Dereference Empty(!) Array by tangent
in thread Dereference Empty(!) Array by lukestyle

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