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my $tree = [ 1
           , [ 2
...
              : $nodefn->( $_, $depth );
    }
}

sub count_nodes {
    my $count;
...

print count_nodes($tree), $/;
# 7

sub print_node {
    my ($leaf, $depth) = @_;
    print "  " x ($depth - 1), "* $leaf", $/;
}

traverse( \&print_node, $tree );
# * 1
...
#       * 5
#     * 6
# * 7

sub bfs_traverse {
    my ($nodefn, $tree, $depth) = @_;
...
    bfs_traverse( $nodefn, [map @$_, @branches], $depth )
        if @branches;
}

bfs_traverse( \&print_node, $tree );
# * 1
...
#     * 4
#     * 6
#       * 5