The point is that $#a should be faster than @a-1. That's not what you are testing. You have have an extra operation in your $#a test instead of having one less.

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw(cmpthese);

our @a = qw( Once upon a time in a galaxy far far away );

cmpthese(-3, {
    cnt_minus_1 => 'my $x = @a-1;',
    last_idx    => 'my $x = $#a;',
});
[download]

                 Rate    last_idx cnt_minus_1
last_idx    3225597/s          --        -25%
cnt_minus_1 4308243/s         34%          --
[download]