This is my best guess. =)

#!/usr/bin/perl

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

my @a = qw(1 2 3 4 5 6 7 8 9);

my $h = {}; my $t = $h;
map { ($t = $t->{$_} = ($_ eq $a[-1])? undef : {})  } @a;

print Dumper($h);
[download]

But as always, use at your own risk.

Cheers!
--habit