Personally I think that I wouldnt use the Readonly module. Partially because what it does is done elsewhere, and i'd be more likely to look there, (Hash::Util for instance would be where i would look for locked hashes). But also because the semantics of readonlyness IMO isnt all that clear. For instance should the readonlyness transfer on assignment? (It doesn't).

The other thing that makes me wonder, is why does the module go to such extreme lengths? Making a constant scalar (which seems to me to be a common task mentioned in this list) is pretty easy without it:

  our $constant;
  *constant=\"this string is a constant";
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Which might not be self documenting, but it solves 99% of the problem domain in a way that doesnt require modules and should work everywhere.