The problem is that you are including the time taken to construct the original (anonymous) array.

Try this and see how you get on (the timings shown are AS 5.10.1 under Vista 64-bit):

#!/user/bin/perl
use strict;
use warnings;
use Benchmark qw[ cmpthese ];

sub sum_copy {
    my @numbers = @_;
    my $sum = 0;
    foreach my $num (@numbers){ $sum += $num }
    $sum;
}

sub sum_ref {
    my ($numbers_ref) = @_;
    my $sum = 0;
    foreach my $num (@$numbers_ref){ $sum += $num };
    $sum;
}

our $N //= 1e6;

my @numbers = 1 .. $N;

cmpthese -3, {
    copy => sub{ my $sum = sum_copy( @numbers ); },
    ref  => sub{ my $sum = sum_ref( \@numbers ); },
};

__END__
C:\test>junk28 -N=1e4
       Rate copy  ref
copy 3.92/s   -- -59%
ref  9.67/s 146%   --

C:\test>junk28 -N=1e5
       Rate copy  ref
copy 3.95/s   -- -59%
ref  9.62/s 144%   --

C:\test>junk28 -N=1e6
       Rate copy  ref
copy 3.92/s   -- -59%
ref  9.66/s 146%   --
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