Try this:

use Data::Dumper;
use strict;

sub insert {
    my $domain_hash = shift;
    my @domain_chain = @_;
    my $parent = $domain_hash;

    my $cur_node;
    while ($cur_node = shift @domain_chain) {
        if (exists($parent->{$cur_node})) {
             $parent = $parent->{$cur_node};
        } else {
             do {
                 $parent->{$cur_node} = {};
                 $parent = $parent->{$cur_node};
             } until (!($cur_node = shift @domain_chain));
        }
    }
}

my $domain_hash = {};
insert($domain_hash, "www", "perlmonks", "com");
insert($domain_hash, "www", "perlmonks", "org");
insert($domain_hash, "www", "google", "com");

print Dumper($domain_hash);
[download]