use warnings;
use strict;
use Data::Dumper;

my @keys="a".."d";
my %hash=();
my $href=\%hash;

my $lastkey= pop @keys;

for my $key (@keys) {
  $href =
    $href->{$key} //= {} ;
}

$href->{$lastkey} = "";

print Dumper \%hash;