G'day mpersico,
Whenever you post a question here that's asking for information related to a module, please ensure you specify which module you are referring to. Vague references to fonts and widgets (as you have here) give no hint as to which of the many Perl GUI modules you're using. Providing a link to the module is even better — see What shortcuts can I use for linking?.
I noted a few of your past nodes asked about Tk. Assuming that's what you're still using, the information you want can be found in the Tk Font documentation. This lists a number of methods which seem to do what you want, e.g. fontMetrics(), fontMeasure(), etc.