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.

Think of it like front desk security issuing outsiders with a visitor's pass. If then you chose to leave them alone in the vault for a while, that is down to you.

• When data comes in to your program from a (potentially) unsafe source it gets marked.
• If you attempt to use that data before you've taken some action to validate it; Perl will yell at you.
• But if Perl never allowed you to remove the mark; you would never be able to use that data.
• And there is no way for Perl to determine whether what you have done has rendered that data "safe". Only you can make that decision.

So perl give you the tools; if you choose to use them incorrectly, that is down to you.