here some basics for union, intersection and diff of two arrays. The keys and values of hashes can be treated as arrays.

> 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
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UPDATE: hmm maybe should be added that exists also works with hashes:

grep { exists $a{$_} } keys %b     # keys intersection

and with refs it's

grep { exists $ar->{$_} } keys %$br     # keys intersection