A Better reason that I find for using Readonly is that is makes your code easier to understand. If you do not have Readonly with XS compiled then you take a performance hit but you generally only see that using a Readonly hash.

Another Advantage Readonly has over use constant is that you can declare a hash, you cannot do that with use constant.

I was pretty sure you could declare a hash with use constant.. or well, a hash reference anyways:

use constant HASH_REF => {
   constant_key => 'constant_value',
   ...
};
[download]

Then use it like you would any hashref ie:
print HASH_REF->{constant_key}
or
foreach my $key (keys %{ &HASH_REF }) { ... }
or the even weirder:
foreach my $key (keys %{ HASH_REF() }) { ... }

You can declare it, yes, but it's not read only. The reference itself remains constant, but the data that it's referring to can be changed:

use constant HASH_REF => { key => 'value' };
HASH_REF->{new_key} = 'new value';
HASH_REF->{key} = 'modified';
use Data::Dumper;
print Dumper HASH_REF;
[download]