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
Inside the sub, before the map @_ = 1 2 3 4 5 6 7 8 9 10
Inside the sub, after the map @arr: 2 4 6 8 10 12 14 16 18 20
Use of uninitialized value $_[0] in join or string at foo line 11.
Use of uninitialized value $_[3] in join or string at foo line 11.
Use of uninitialized value $_[6] in join or string at foo line 11.
Use of uninitialized value $_[9] in join or string at foo line 11.
Inside the sub, after the map @_ =  _ Use of uninitialized value $_[0] in join or string at foo line 11.
  _ Use of uninitialized value $_[3] in join or string at foo line 11.
  _ 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