I always thought map just returns copies of the values?

my @a = qw(1 2);
my @b = \(@a);
my @c = \(map $_,@a);
print "Addresses do not match$/" unless "@b" eq "@c";
__END__
Addresses do not match
[download]

Of course, changing your map to:

${$_}++ for map \(@$_),@{$a};
[download]

Will make it work as expected.