$ perl fringe.pl
---
[[1, [[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]]]
[1, [[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]]
leaf: 1
---
[[[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]]
[[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]
[2, 3, [4, [[[5, 6], [7, 8]], [9, 10]]]]
leaf: 2
---
[3, [4, [[[5, 6], [7, 8]], [9, 10]]]]
leaf: 3
---
[[4, [[[5, 6], [7, 8]], [9, 10]]]]
[4, [[[5, 6], [7, 8]], [9, 10]]]
leaf: 4
---
[[[[5, 6], [7, 8]], [9, 10]]]
[[[5, 6], [7, 8]], [9, 10]]
[[5, 6], [7, 8], [9, 10]]
[5, 6, [7, 8], [9, 10]]
leaf: 5
---
[6, [7, 8], [9, 10]]
leaf: 6
---
[[7, 8], [9, 10]]
[7, 8, [9, 10]]
leaf: 7
---
[8, [9, 10]]
leaf: 8
---
[[9, 10]]
[9, 10]
leaf: 9
---
[10]
leaf: 10
---
[]

##</code><code>##

$ perl fringe.pl
---
tree: [1, [[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]]
rtrees: []
traverse L
  tree: 1
  rtrees: [[[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]]
leaf: 1
---
tree: [[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]
rtrees: []
traverse L
  tree: [2, 3]
  rtrees: [[4, [[[5, 6], [7, 8]], [9, 10]]]]
traverse L
  tree: 2
  rtrees: [[4, [[[5, 6], [7, 8]], [9, 10]]], 3]
leaf: 2
---
tree: 3
rtrees: [[4, [[[5, 6], [7, 8]], [9, 10]]]]
leaf: 3
---
tree: [4, [[[5, 6], [7, 8]], [9, 10]]]
rtrees: []
traverse L
  tree: 4
  rtrees: [[[[5, 6], [7, 8]], [9, 10]]]
leaf: 4
---
tree: [[[5, 6], [7, 8]], [9, 10]]
rtrees: []
traverse L
  tree: [[5, 6], [7, 8]]
  rtrees: [[9, 10]]
traverse L
  tree: [5, 6]
  rtrees: [[9, 10], [7, 8]]
traverse L
  tree: 5
  rtrees: [[9, 10], [7, 8], 6]
leaf: 5
---
tree: 6
rtrees: [[9, 10], [7, 8]]
leaf: 6
---
tree: [7, 8]
rtrees: [[9, 10]]
traverse L
  tree: 7
  rtrees: [[9, 10], 8]
leaf: 7
---
tree: 8
rtrees: [[9, 10]]
leaf: 8
---
tree: [9, 10]
rtrees: []
traverse L
  tree: 9
  rtrees: [10]
leaf: 9
---
tree: 10
rtrees: []
leaf: 10
---
tree: undef
rtrees: []

##</code><code>##

#!/usr/bin/perl
use strict;
use Data::Dump 'dd';

my $tree =
[
    1, [[2, 3], [4, [[[5, 6], [7, 8]], [9, 10]]]]
];

#my $T = get_tree_iterator($tree);
#my $T = get_tree_iterator_explicit($tree);
#my $T = get_tree_iterator_2($tree);
my $T = get_tree_iterator_2_explicit($tree);

while (my $leaf = $T->()) {
    print "leaf: $leaf\n";
}

sub get_tree_iterator {
    my @stack = (shift);
    return sub {
        print "---\n";
        dd \@stack;
        while (@stack and ref $stack[0] eq 'ARRAY') {
            unshift @stack, @{shift @stack};
            dd \@stack
        }
        return shift @stack;
    }
}

sub get_tree_iterator_explicit {
    my @stack = (shift);
    return sub {
        print "---\n";
        dd \@stack;
        while (@stack and ref $stack[0] eq 'ARRAY') {
            print "  split tree into left & right\n";
            unshift @stack, @{shift @stack};
            dd \@stack
        }
        return shift @stack;
    }
}

sub get_tree_iterator_2 {
    my @rtrees = (shift);
    my $tree;
    return sub {
        $tree = pop @rtrees;
        print "---\n";
        print "tree: "; dd $tree;
        print "rtrees: "; dd \@rtrees;
        ($tree, $rtrees[@rtrees]) = @$tree while ref $tree;
        return $tree;
    }
}

sub get_tree_iterator_2_explicit {
    my @rtrees = (shift);
    my $tree;
    return sub {
        $tree = pop @rtrees;
        print "---\n";
        print "tree: "; dd $tree;
        print "rtrees: "; dd \@rtrees;
        while (ref $tree) {
            print "traverse L\n";
            ($tree, $rtrees[@rtrees]) = @$tree;
            print "  tree: "; dd $tree;
            print "  rtrees: "; dd \@rtrees;
        }
        return $tree;
    }
}