Ah but your templating system is using global variables for the substitution, which is why it doesn't work with a hash. If you see "$(things)" you try and replace it with the contents of $things. And that's using symbolic references, which is a bad idea. See these 3 posts by dominus for an explanation why symbolic references are a very bad idea, in general. The moral of it is that it's very easy for them to clash with vital variables for your program. It's like driving a car with your passengers seated on your engine.

Why don't you build your templating system around a hash? Store every value you ever want to use in one hash, let's call it %var, and do the substitution like this:

 $fld =~ s/\$(\w+)/$var{$1}/g;
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Dead easy. You can simply use %params as your hash, or copy the desired values to %var first:

 %var = (%params, foo => 'this is a new or changed field', bar => 'and
+ another', greet => 'Hello');
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If your template contains "$(greet)" it'll get replaced by "Hello".