Im actually thinking the question was.

Which is faster?

$num = scalar(@array);
# or 
$num = $#array;
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And as someone pointed out $#array isnt acurate as to the actual number of elements in the array, so I guess you could simply compare the op trees of the two. Or I guess perform 3 benchmarks,
1) plain
2) scalar(@array) - 1
3) $#array + 1;

/* And the Creator, against his better judgement, wrote man.c */