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;
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