$ 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
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[8, [9, 10]]
leaf: 8
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[[9, 10]]
[9, 10]
leaf: 9
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[10]
leaf: 10
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[]