my %partitions;
  while (
  ...
  my @cols = split(',', $line);
  my $pname = "$cols[0]:$cols[2]";
  $partitions{$pname} ||= [];
  push @{$partitions{$pname}}, $cols[3];
  }

##</code><code>##

  use List::Util qw(sum);

  for my $pname (sort keys %partitions) {

     my $count = scalar @{$partition{$pname}};
     my $sum = sum @{$partition{$pname}};
     my $mean = $sum/$count;
     
     # std dev is left as an exercise
  }