#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11146593
use warnings;
use List::AllUtils qw( zip_by sum );

my @mapped = ("Account Entity Unit Jan Feb Mar",
            "Account01 Entity01 Unit01 1 2 3",
            "Account01 Entity01 Unit01 4 5 6",
            "Account01 Entity01 Unit01 7 8 9",
            "Account02 Entity02 Unit02 10 11 12");

local $_ = join '', map "$_\n", @mapped;
1 while s/^((?:\S+\s+){3})\b\K(.*\n)((?:.*\n)*)\1(.*\n)/
  join(' ', zip_by { sum @_ } map [ split ], $2, $4) . "\n$3"/me;
s/(.*\n)\K/ $1 =~ tr| \n|-|cr /e;
printf scalar( ('%-11s' x 6 . "\n") x tr/\n// ), split;

##</code><code>##

Account    Entity     Unit       Jan        Feb        Mar        
-------    ------     ----       ---        ---        ---        
Account01  Entity01   Unit01     12         15         18         
Account02  Entity02   Unit02     10         11         12