this is quite close but then I'd need to go throught one of the lists one more time :-/

I think you have to :)

#!/usr/bin/perl --
use strict; use warnings; use Data::Dump;

my @source=(1, 2, 3, 4, 5, 7);
my @target=(0, 1, 3, 4, 6);

my (@union, @intersection, @difference);

my %count = ();

foreach my $element (@source, @target) {
    $count{$element}++
}
foreach my $element (keys %count) {
    push @union, $element;
    push @{ $count{$element} > 1 ? \@intersection : \@difference }, $e
+lement;
}

dd \@source, \@target;
dd \@union;
dd \@intersection;
dd \@difference;
my @values_in_target_not_in_source = do {
    my %fuf;
    @fuf{ @union } = undef;
    delete @fuf{ @target };
    keys %fuf;
};

my @values_in_source_not_in_target = do {
    my %fuf;
    @fuf{ @union }  = undef;
    delete @fuf{ @source  };
    keys %fuf;
};
dd \@values_in_target_not_in_source , \@values_in_source_not_in_target
+ ;
__END__
([1 .. 5, 7], [0, 1, 3, 4, 6])
[6, 4, 1, 0, 3, 7, 2, 5]
[4, 1, 3]
[6, 0, 7, 2, 5]
([7, 2, 5], [6, 0])
[download]

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