Your method requires that all data to be in memory at once, but it's simple to refactor it so that's not the case. 

my ($cnt, $sum, $squ);
while (my ($d) = $iter->()) {
   $cnt++;
   $sum += $d;
   $squ += $d * $d;
}

my $mean = $sum / $cnt;
my $var = $squ + -2*$mean*$sum + $mean*$mean*$cnt;
my $std  = sqrt($var/$cnt);

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while (my ($d) = $iter->()) can be replaced with any loop, including a file reading loop or a database fetching loop.

Update: If you don't need $var anywhere else, the last two lines can be simplified to 

my $std  = sqrt($squ/$cnt - $mean*$mean);

[download]

In reply to Re^2: Correlation plots by ikegami
in thread Correlation plots by Win

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