Two problems with scalar split: 1) pre-5.12, it will globber @_. 2) it's incorrect:

print scalar split /\n/ for "1\n2\n3\n\n";  # Prints 3 instead of 4
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Using a negative third argument doesn't solve the problem:

print scalar split /\n/, $_, -1 for "1\n2\n3\n\n";  # Prints 5
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you'd have to subtract 1, but only if the string ends in a new line - so you end up looking at the end as well.

As for the complexity:

print tr/\n// + !/\n\z/;
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even gives the correct results in list context.