In order to untaint, you need to capture what you want to keep anyway.

For the sake of being argumentative, no, you don't have to. This is Perl though, you're free to shoot yourself in the foot. You could f.ex s/// everything you want to disallow, then capture /(.*)/s.

But obviously, the untaint mechanism was chosen to encourage "allow what you will accept" logic.

Makeshifts last the longest.