use List::Util 'reduce';
use Carp 'carp';

# a.tree => b.tree => c.tree => ... z.value

sub put_element {
    'HASH' eq ref( my $root = shift() )
        or carp "put_element( \ %..., [ KEY ], $val ) must be a hash reference";
    'ARRAY' eq ref( my $keys = shift() )
        or carp "put_element( \ %..., [ KEY ], $val ) must be given an array reference";
    my $value = shift;

    my $leaf = reduce { $a->{$b}{'tree'} ||= {} } $root, @{$keys}[ 0 .. $#$keys - 1 ];
    $leaf->{$keys->[-1]}{'value'} = $value;
}