sub average {
@_ == 1 or die ('Sub usage: $average = average(\@array);');
my ($array_ref) = @_;
my $sum;
my $count = scalar @$array_ref;
foreach (@$array_ref) { $sum += $_; }
return $sum / $count;
}

##</code><code>##

sub average {
    ref( my $array_ref = $_[ 0 ] ) eq 'ARRAY'
        or die 'Sub usage: $average = average(\@array);';
    my $sum;
    $sum += $_ for @$array_ref;
    return $sum / @$array_ref;
    }