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Before calling the sub test_this @arr: 1 2 3 4 5 6 7 8 9 10
Inside the sub, before the map @arr: 1 2 3 4 5 6 7 8 9 10
...
_ Use of uninitialized value $_[6] in join or string at foo line 11.
After calling the sub test_this @arr: 2 4 6 8 10 12 14 16 18 20