$ 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: []