Why o Why don't you use Text::Template or one of the 263 other template modules on CPAN? Do you think the World need another template system?
Seriously, you will start with a simple template mechanism, then you will add function support, then something else, then one more feature... and before you know it you have built a package with most of the features of the CPAN ones, just in a much more clunky way and without their robustness.

Please do yourself a favor and use a module!