#!/usr/bin/perl

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

my %group_by;
my @titles = split ' ', <DATA>;
while( <DATA> )
  {
  my @parts = split;
  my %hash = zip_by { @_ } \@titles, \@parts;
  for ( qw( Jan Feb Mar ) )
    {
    $group_by{ join ' ', @hash{ qw( Account Entity Unit ) } }{ $_ } += $hash{ $_ };
    }
  }
use Data::Dump 'dd'; dd \%group_by;

__DATA__
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

##</code><code>##

{
  "Account01 Entity01 Unit01" => { Feb => 15, Jan => 12, Mar => 18 },
  "Account02 Entity02 Unit02" => { Feb => 11, Jan => 10, Mar => 12 },
}