The Date::Manip module provides some easy methods for dealing with this sort of thing on a pretty high level. It's slower than Date::Calc but for your problem it sounds like a one-shot calculation up front. For example you could do this:

  $next_date = DateCalc($cur_date,"+ 1 month");
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and it just does the right thing.

In your case, though, it sounds like you'd be looking backwards, so you might be using more along the lines of:

  $activation_date = DateCalc('today', '- 1 month');
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then checking your database for folks that signed up then.

Matt