Apart from List::Utils that others already mentioned I'd like to add that if you want to do union, intersection or difference you should most likely be using a hash, not an array anyway. A hash with empty values, but a hash nevertheless.

# is element $a in the set?
exists $set{$a}
# remove $a from the set
delete $set{$a};
# add $a to the set
$set{$a} = undef;
# get the list of elements in the set
keys %set
# create the union
%union = (%set1, %set2);
# create the intersection
for (keys %set1) {
  $intersect{$_} = undef if exists $set2{$_};
}
# create the difference
for (keys %set1) {
  $diff{$_} = undef unless exists $set2{$_};
}
for (keys %set2) {
  $diff{$_} = undef unless exists $set1{$_};
}
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This will be much quicker than anything you could build with arrays.