#! /usr/bin/perl
use strict;
use warnings;

my @keys = qw( foo bar );

my $x = {};

{
    my $t = $x;
    for my $i ( 0 .. $#keys ) {

        my $v = $i == $#keys ? join('',@keys) : {};
        $t = $t->{ $keys[$i] } = $v;
    }
}

use Data::Dumper;
print Dumper( $x );