Maybe I'm missing the point here, but is there some reason that you don't use the $#$ construct? You even have it in your comments.

#!/usr/bin/env perl 
use strict;
use warnings;
use Test::More tests => 3;

my $data = ['alpha', 'beta'];
my $count = $#$data + 1;
is ($count, 2, 'With elements');

$data = [];
$count = $#$data + 1;
is ($count, 0, 'Without elements');

$data = undef;
$count = $#$data + 1;
is ($count, 0, 'As undef');
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By my reading of your post this seems to satisfy all your criteria and is both clear and concise. HTH.