I use Statistics::Descriptive:

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dump;
use Statistics::Descriptive;

my $stat = Statistics::Descriptive::Full->new();

my @act_ret = ( 1, 2, 3, 4 );

$stat->add_data(@act_ret);

dd $stat->standard_deviation();

__END__

karls-mac-mini:monks karl$ ./deviation.pl
1.29099444873581
[download]

Seems to work ;-)

See also

Regards, Karl