I'll take the opportunity to plug reduce again:

use List::Util qw(reduce);
use Data::Dumper;

my @c = (1, 2, 3, 4);
my %d;

reduce { $a->{$b} ||= {} } \%d, @c;
print Dumper \%d;
[download]

yields:

$VAR1 = {
          '1' => {
                   '2' => {
                            '3' => {
                                     '4' => {}
                                   }
                          }
                 }
        };
[download]

If the value of the terminating key needs to be something else than {}:

(reduce { $a->{$b} ||= {} } \%d, @c[0..$#c-1])->{$c[$#c]} = 42;
print Dumper \%d;
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which is slightly uglier, but will yield:

$VAR1 = {
          '1' => {
                   '2' => {
                            '3' => {
                                     '4' => 42
                                   }
                          }
                 }
        };
[download]