when I first started reading about taint mode, I expected that it would identify every single instance of tainted variable and force me to look at it explicitly.

It does! What it cannot do -- which you seem to be expecting -- is decide whether you looked closely enough.

I understand that it's my responsability to make sure I have looked at the input closely enough. My issue is that Perl tries to "guess" when I have looked at the the input ("gee, the programmer captured some match groups from a regexp match on that input, so it MUST mean that he sanitized it"), instead of letting me tell it when I think I have looked at it closely enough (for example, but invoking a method untainted() on a variable).

Using your front desk metaphor, suppose I am a security guard patrolling the corridors of a building. As I go through the front gate in the morning, I notice my front desk colleague making eye contact with a visitor. Later on, I see this visitor wandering the corridors without a pass. Can I assume that this visitor is authorized just because my colleague made eye contact with him? No, of course not!