It's very easy to say "well, you can do taint checking without taint mode on". And yes, you are right. But your comment "such a thing wouldn't likely occur to someone who knows what they are doing." is counter-intuitive.

A good programmer thinks not only of what (s)he is capable of, but also that the code will be maintained in the future -- maybe by someone else. Not using tainting invites three forms of disaster:

1. Mistakes in untainting (this is analogous to why good programmers use strictures)
2. Tainting introduced accidentally during a "quick revision"
3. Tainting introduced by another developer during maintainance.

If all one is doing involves validating a name and email address or some such simplicity, then perhaps tainting isn't absolutely necessary, but when one is dealing with large amounts of data, it is.

For this solution, the performance hit is quite tiny. In large environments where performance would be a significant issue, one would likely be able to make the case with the admin staff to set up a perl -T association.