I'd be happier developing with a function that called Readonly in development and then something like this in production:

sub fake_readonly {
  $_[0] = $_[1];
}
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And now you get most of the advantages of Readonly, but performance is not a problem. (I don't like constant because I'm a big fan of interpolating, and I often have to check whether someone has a constant or function because I'm afraid that it will try to suck up arguments.)