the sort thing that doesn't work

$ cat test.pl 
my $a = join "", <>;
print "\n";

$h = {
    'thing1' => { 'Namespaces' => [ qw<b d f> ] },
    'thing2' => { 'Namespaces' => [ qw<a c h> ] },
};

for (map { $h->{$_}{Namespaces}[0] } keys %{$h}) {
    print $_;
    print "\n";
}

# Sort alphabetically on the first namespace
sub by_namespace {
    $h->{$a}{Namespaces}[0] cmp $h->{$b}{Namespaces}[0];
}
my @managers = sort by_namespace keys %{$h};
for (@managers) {
    print $_, "\n";
}
$ perl test.pl 
^D
b
a
thing1
thing2