You are measuring the time for creating the array reference. Don't do: print sum([1..100_000]); Do something like this:

#! /usr/bin/perl
# vim:ts=4 sw=4 sts=4 et nu fdc=3:
use strict;
use warnings;
use Benchmark qw( cmpthese );



#> sub routines
#> -------------------------------------------------------------------
+---------

sub sum_list {
    my @numbers = @_;

    my $sum = 0;
    for my $num ( @numbers ) {
        $sum += $num;
    } 

    return $sum;
}

sub sum_by_ref {
    my ( $numbers_ref ) = @_;

    my $sum = 0;
    for my $num ( @$numbers_ref ) {
        $sum += $num;
    } 

    return $sum;
}

#> main script
#> -------------------------------------------------------------------
+---------

my @numbers = 1 .. 1_000_000;
cmpthese( -1, {
    'sum_list'   => sub { sum_list(@numbers);    },    
    'sum_by_ref' => sub { sum_by_ref(\@numbers); },    
});


__END__
[download]

             Rate   sum_list sum_by_ref
sum_list   4.42/s         --       -45%
sum_by_ref 8.00/s        81%         --
[download]

I hope my point got clear ;o)

Addendum: Tested with creating list and reference directly within function call:

cmpthese( -1, {
    'sum_list'   => sub { sum_list(1..1_000_000);     },    
    'sum_by_ref' => sub { sum_by_ref([1..1_000_000]); },    
});

Result:
             Rate   sum_list sum_by_ref
sum_list   4.13/s         --        -9%
sum_by_ref 4.55/s        10%         --
[download]