Here are some things you might consider depending upon what it is you really want to achieve.

1. Make the results different for the user (then the problem goes away). Why not tell the user that the reason for the error is that foo is more than 10 - isn't this useful information to them?
2. If you want to continue to hide the specific error from the user for some reason then simply add in a differing response for your own use which the user won't see. This might be:
   1. Throw different exceptions
   2. Raise different warnings
   3. Return different values from the subroutine
3. Use Devel::Cover to ensure that your test suite is exercising all the possibilities. You do have a test suite, don't you?