Here you go. This one's free! ;)

sub standard_deviation {
  # declared all at once
  my ($mean, $std_dev) = (0,0);

  # idioms are handy -- I like the postfix 'for'
  # and '$x = $x + $y' can be written as '$x += $y'
  $mean += $_ for @_;

  # no need for $size, it's used once, and is @_

  # again, using the 'A = A OP B' => 'A OP= B' trick
  $mean /= @_;

  # appending to arrays is best done with push()
  # @big_array = (@big_array, $x) can get slow :(

  # for (@_) {
  #   push @diffs, ($_ - $mean)**2;
  # }

  # that can be rewritten as a map()
  # @diffs = map { ($_ - $mean)**2 } @_;

  # but we only use @diffs ONCE, so we don't need it

  $std_dev += ($_ - $mean)**2 for @_;

  # and $size was removed, so we use @_ again
  return $std_dev / @_;
}
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When it comes down to it, the function becomes:

sub standard_deviation {
  my ($mean, $std_dev) = (0,0);

  $mean += $_ for @_;
  $mean /= @_;

  $std_dev += ($_ - $mean)**2 for @_;
  return $std_dev / @_;
}
[download]

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
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