I note two things: you only want the number of elements in the intersection and union, not the actual intersection and union; and your method suggests that there are no duplicates in the sets.
So you don't need to find the union, explicitly. It is just the total number in I + J, minus the intersection.
sub get_int_uni2 {
my ($in, $jn) = @_;
my %i;
@i{@$in} = ();
my $int = grep exists $i{$_}, @$jn;
return ($int, @$in + @$jn - $int);
}
Sadly, I don't see any real improvement in performance (thanks to BrowserUK for the benchmarkery).
Caution: Contents may have been coded under pressure.