Same with "nicer" printout.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11146593
use warnings;

my @copy = 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");

my %group_by;
my @titles = split ' ', shift @mapped;

foreach (@mapped) {
    my %hash; @hash{ @titles } = split;   #don't get it NOTE makes a h
+ash with titles as keys and fields of current line as values
    for(qw(Jan Feb Mar)){
        $group_by{join ' ', @hash{qw(Account Entity Unit)}}{ $_ } += $
+hash{ $_ };    #don't get it either NOTE does the SUM function that y
+our SQL is doing
    }
}
#use Data::Dump 'dd'; dd \%group_by;

# if you want a "nice" printout

open my $incolumns, '|-', 'column -t' or die;
print $incolumns "$copy[0]\n", "$copy[0]\n" =~ s/\w/-/gr; # separator 
+line
foreach( sort keys %group_by )
  {
  print $incolumns "$_ @{ $group_by{ $_ } }{qw(Jan Feb Mar)}\n";
  }
close $incolumns;
[download]

Outputs:

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