use strict;

sub nested_value {
  my $href = shift;
  my $keys = shift;

  my $last_key = pop @$keys;

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

  if (@_) {
    $href->{$last_key} = shift;
  }

  return $href->{$last_key};
}

my %hash;

nested_value(\%hash, [qw/one two three/], 1);
print nested_value(\%hash, [qw/one two three/]), "\n";

use Data::Dumper;

print Dumper \%hash;