I'm sure this isn't the answer you were looking for, but honestly, look in CPAN for general things like this. You'll save yourself a lot of missteps. For instance, Statistics::Descriptive seems to be just what you wanted.

use Statistics::Descriptive;
my $s = Statistics::Descriptive->new();
$s->AddData(22, 17, 29, 39, 20, 32);
print "The standard deviation is ", $s->StandardDeviation();
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That's how I'd do it, anyway.

HTH