use strict;

sub put {
    my ($hash_ref, $main_key, $sub_key, $value) = @_;
    if (!exists($hash_ref->{$main_key})) {
        $hash_ref->{$main_key} = {};
    }
    $hash_ref->{$main_key}->{$sub_key} = $value;
}

sub display {
    my $hash_ref = shift;

    my ($main_key, $sub_key);

    foreach $main_key (keys %{$hash_ref}) {
        foreach $sub_key (keys %{$hash_ref->{$main_key}}) {
            print "{$main_key, $sub_key} = " . $hash_ref->{$main_key}-
+>{$sub_key}, "\n";
        }
    }
}


my $hash_ref = {};
put($hash_ref, "one", "key1", "value1");
put($hash_ref, "one", "key2", "value2");
put($hash_ref, "two", "key1", "value1");
put($hash_ref, "two", "key3", "value3");
display($hash_ref);