Somewhat cleaner recursion here..

use Data::Dumper;

my $a;

sub x{
  my $ar=$_[1];
  my $v=shift @$ar or return $_[0]=undef;
  x($_[0]->{$v} , $ar) 
};

x($a,[5,3,4,5,6]); 

print Dumper \$a;
[download]

Prints..

$VAR1 = \{
            '5' => {
                     '3' => {
                              '4' => {
                                       '5' => {
                                                '6' => undef
                                              }
                                     }
                            }
                   }
          };
[download]