#! /usr/bin/perl
use warnings;
use strict;

use List::Util qw{ sum };
use Data::Table;

my $t = new Data::Table(
 [
 ['a', 1000, 2000, 3000, 200,500],
 ['b', 2000, 1000, 1000, 700,800],
 ['c', 3000, 3000, 3000, 5,7],
 ],
 ['Name', 'value1', 'value2', 'value3', 'value4', 'value5'], 0);

$t->addCol(undef, 'v1-3');
$t->colsMap(sub { $_->[-1] = sum(@$_[1 .. 3]) / 3 });

$t->addCol(undef, 'v4,5');
$t->colsMap(sub { $_->[-1] = sum(@$_[4, 5]) / 2 });


print $t->csv;
[download]

Or, more DRY:

sub set_avg {
    my @cols = @_;
    sub { $_->[-1] = sum(@$_[@cols]) / @cols }
}

$t->addCol(undef, 'v1-3');
$t->colsMap(set_avg(1 .. 3));

$t->addCol(undef, 'v4,5');
$t->colsMap(set_avg(4, 5));
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]