That's nice. ++

However, you don't need to check if an element exists in the current array:

    my @ary1 = (1, 2, 3, 4, 5, 6, 7);
    my @ary2 = (6, 7, 8, 9, 10, 11);

    my %h1;
    my %h2;
    @h1{@ary1} = undef;
    @h2{@ary2} = undef;

    my @dupes;
    @ary1 = grep {not exists $h2{$_} or not push @dupes, $_} @ary1;
    @ary2 = grep {not exists $h1{$_}                       } @ary2;

    print "ary1 is @ary1 \n";
    print "ary2 is @ary2 \n";
    print "dupes were @dupes \n";
[download]

Or even just this:

    @dupes = grep {    exists $h2{$_}} @ary1;
    @ary1  = grep {not exists $h2{$_}} @ary1;
    @ary2  = grep {not exists $h1{$_}} @ary2;
[download]

--
John.