This seems to do the trick

use Data::Dumper;

my @a = split ' ', q[foo bar baz quux];
my $le = my $h = {};
for(0 .. $#a - 1) {
  $le->{ $a[$_] } = { $a[$_ + 1] => {} };
  $le = $le->{ $a[$_] };
}

print Dumper $h;

__output__

$VAR1 = {
          'foo' => {
                     'bar' => {
                                'baz' => {
                                           'quux' => {}
                                         }
                              }
                   }
        };
[download]

Although are you sure this is the trick you want to be performing?
HTH

_________
broquaint