my $hash_reference = {};  # empty hash ref for now

my $a = 'fee';
my $b = 'fie';
my $c = 'foe';

$hash_reference->{$a}{$b}{$c} = 'fum';
# same as...
$hash_reference->{'fee'}{'fie'}{'foe'} = 'fum';
# same as...
$hash_reference->{fee}{fie}{foe} = 'fum';

print $hash_reference->{fee}{fie}{foe};