Using Readonly::XS is nice and all, but I generally just use $NAMING_STYLE to tell the difference. It's not foolproof, but it's simple. (I'm sure that now I will go write code that breaks itself by assigning to a fake constant.)

What surprises me is that lack of mentions of putting those data into configuration. I'm not sure what kind of efficiency you're working for, but things like constants are often best moved to options or configuration. You can change them all you like without having to worry about rev'ing the code.