The sort is eating up most of your time here. You want something to show the expense of the stack manipulations in passing the array directly. How about returning a count of the elements?

use strict;
use Benchmark;

my $len  = 4000;
my @data;
push(@data,rand()) for (0..$len);

timethese(90000, {
    'by_array' => sub { my $count = count_array(      @data ); },
    'by_ref'   => sub { my $count = count_array_ref( \@data ); },
});

sub count_array
{
    return scalar @_;
}

sub count_array_ref
{
    return scalar @{ $_[0] };
}
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