Here is a recursive solution written more for clarity than conciseness. It'll give a basic idea anyway:

#!/user/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my @array = qw(a b c d e f g h i 123);
my $recurse_level = 0;

sub to_hash {
  my $hash_ref = shift;
  my @array = @_; 
  my $array_count = @array;
  die "Need at least 2 elements in original array" if (0 == $recurse_l
+evel && 2 > $array_count);
  my $index;
  if (2 < $array_count) {
    $index = shift @array;
    $recurse_level++;
    $hash_ref->{$index} = to_hash({},@array);
  }   
  else {
    $index = shift @array;
    $hash_ref->{$index} = shift @array;
  }
  return $hash_ref;
}

my $hash_ref = to_hash({},@array); 

print Dumper($hash_ref);
[download]

Output:

$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => {
                                     'd' => {
                                              'e' => {
                                                       'f' => {
                                                                'g' =>
+ {
                                                                      
+   'h' => {
                                                                      
+            'i' => '123'
                                                                      
+          }
                                                                      
+ }
                                                              }
                                                     }
                                            }
                                   }
                          }
                 }
        };
[download]

Update

Triple bonus points to whomever modifies this code to allow for more complex data structures to be created by feeding it multiple arrays - the current code only returns a hash for /the/ array passed to it. I know why, but I can figure out right now how to fix it...if I figure it out, I'll update the code.