use List::Compare;

my @Llist = ('A','B','C','D','E');
my @Rlist = (            'D','E','F','G','H','I','J');

my $lc = List::Compare->new(\@Llist, \@Rlist);

# If I read your post correctly,
# this is what you want.
my @LorRonly = $lc->get_symmetric_difference;
print join ", ", @LorRonly;

# Also does...
my @intersection = $lc->get_intersection;
my @union        = $lc->get_union;

# And much, much more.