Standard Deviation in Perl

I recently wanted to figure out the average and standard deviation of a set of values in a perl script. Here's the script I created to do just that:

#!/usr/bin/perl

use strict;
use warnings;

my @my_array = (1,6,9,2,5,12,1);

my $average = average(@my_array);
my $std_dev = std_dev($average, @my_array);

printf "avg=%.2f, st. dev=%.2f\n", $average, $std_dev;

sub average {
        my (@values) = @_;

        my $count = scalar @values;
        my $total = 0; 
        $total += $_ for @values; 

        return $count ? $total / $count : 0;
}

sub std_dev {
        my ($average, @values) = @_;

        my $count = scalar @values;
        my $std_dev_sum = 0;
        $std_dev_sum += ($_ - $average) ** 2 for @values;

        return $count ? sqrt($std_dev_sum / $count) : 0;
}