> perl -de0
...
  DB<1> @a=0..6
 
  DB<2> @b=3..9
 
  DB<3> print grep { exists $a[$_] } @b     # intersection
3456
  DB<4> print grep { ! exists $a[$_] } @b   # missing in @a
789
  DB<5> @union{@a,@b}=()
 
  DB<6> print keys %union                   # union
6379281405