I have some hope that the RE engine will be able to do this optimization, but you can create a RE as I did at RE (tilly) 4: SAS log scanner that will match any key, and then do a hash lookup for the substitution. (Make appropriate modifications to allow case insensitivity, and to allow arbitrary punctuation and whitespace where the _ appears. The code there will not work exactly as shown.)

However that way lies insanity. (Trust me. It does.)

If you can, you really want to introduce a standard templating solution early and go with that. See, for instance, Template::Toolkit or Text::Template.