Case1:
AtUnderscore: This_is_a_string,3,5
shift: This_is_a_string
AtUnderscore after shift: 3 5
$_[1]: 5
Use of uninitialized value in concatenation (.) at ./mysplit line 16.
$_[2]: 


Case2:
AtUnderscore: This_is_a_string,3,5
Rubbish: s_i
1: 5
Use of uninitialized value in concatenation (.) at ./mysplit line 34.
2: