How to calculate standard deviation of coverage for each scaffold in a pileup file?

Question

[PollardMD]

A previous member of my research group wrote a perl script that printed scaffold length and mean coverage for each scaffold within a .pileup file created by the samtools mpileup function. I would like to add to this script so that the output file will also have a column for the standard deviation of coverage for each scaffold, but unfortunately I have no experience with perl. How would I go about doing this? Here is the current version of the script, for reference:

my $file = "example.pileup";
my $outfile = "example.threecolumn.txt";


my $ds;


open FPIN, "<".$file or die "Cannot open $file";

while (<FPIN>) {

    die unless (/^([^\t]+)\t[^\t]+\t[^\t]+\t([^\t]+)\t/);

    my ($scaf, $num) = ($1, $2);

    if ($ds->{$scaf}) {
        $ds->{$scaf}->{"count"} = $ds->{$scaf}->{"count"} + 1;
    } else {
        $ds->{$scaf}->{"count"} = 1;
        $ds->{$scaf}->{"sum"} = 0;
    }

    $ds->{$scaf}->{"sum"} = ($ds->{$scaf}->{"sum"} + $num);
}

close FPIN;

open FPOUT, ">".$outfile or die;

foreach my $scaf (keys %$ds) {

    print FPOUT $scaf."\t".($ds->{$scaf}->{"sum"} / $ds->{$scaf}->{"count"})."\t".$ds->{$scaf}->{"count"}."\n";

}

close FPOUT;

Many thanks in advance for any suggestions.