DB<56>  x @a
0  undef
1  1
2  undef
3  3
  DB<57>  $a[5] = undef

  DB<58>  x @a
0  undef
1  1
2  undef
3  3
4  undef.  # gap 
5  undef.  # value
  DB<59>  p exists $a[4]

  DB<60>  p exists $a[5]
1
  DB<61>