my $data = [
   [ 'parent1', 'child1a', 'child2a', undef,	],
   [ 'parent1', 'child1b', 'child2b', 'child3b',],
   [ 'parent1', 'child1c', undef, undef,],
];

##</code><code>##

my @results;
foreach my $row (@$data) {
   push @results, treeify(@$row);
}

print Dumper @results;

sub treeify {
   my @list = @_;
   my $node = shift @list;
   return {} unless $node;
   return {$node => treeify(@list)};
}


##</code><code>##

$VAR1 = {
          'parent1' => {
                         'child1a' => {
                                        'child2a' => {}
                                      }
                       }
        };
$VAR2 = {
          'parent1' => {
                         'child1b' => {
                                        'child2b' => {
                                                       'child3b' => {}
                                                     }
                                      }
                       }
        };
$VAR3 = {
          'parent1' => {
                         'child1c' => {}
                       }
        };