#!/usr/bin/perl

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

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 hash 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 your SQL is doing
    }
}
use Data::Dump 'dd'; dd \%group_by;