@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 ' ', @mapped;

foreach (@mapped) {
	#print $_."\n";
	my @parts = split;
	#print @parts."\n";
	my %hash; @hash{ @titles } = @parts;   #don't get it
	for(qw(Jan Feb Mar)){
		$group_by{join ' ', @hash{qw(Account Entity Unit)}}{ $_ } +=+ $hash{ $_ };    #don't get it either
	}
}
use Data::Dump 'dd'; dd \%group_by;