Just for starters, ??{ . . . } can be used to execute code. That would be bad. There's also nothing stopping future versions of Perl to add some other way to execute code or do other nasty things if you let the search string go through. So even if you're safe now, you might not be safe when you upgrade Perl a few years from now.

Just to note, this is only true if you specifically enable it via use re 'eval';. As for perl suddenly allowing a new way to interpolate code in to a regex and execute it, this is really rather unlikely. If they do, it will definitely only work with a new, specific switch.