tree.pl

use strict;
use warnings;

package Node {
    use Class::Accessor::Lite ro => [qw/value children/], new => 1;
}

# DSL
package main {
    sub node ($;&) { goto \&_node }
    sub _node {
        my ($value, $code) = @_;

        my @children;
        if ($code) {
            my $node_sub = sub ($;&) { push @children => _node(@_) };

            no warnings qw/redefine/;
            local *node = $node_sub;
            use warnings qw/redefine/;

            $code->();
        }

        return Node->new(value => $value, children => \@children);
    }
};

my $tree = node 1, sub {
    node 2, sub {
        node 3, sub {
            node 4;
            node 5;
        };
        node 6;
    };
    node 7;
    node 8, sub {
        node 9, sub {
            node 10;
            node 11;
        };
        node 12;
    };
};