The essential idea behind “taint mode” is that ... “anything and everything that has been provided directly by the user should be presumed to be thoroughly infested with cooties.”

In other words, such values are “tainted.”   (To put it another way... “suspect.”   “Highly doubtful.”   “Not as trustworthy as a value which the program itself has directly generated.”   “Used goods.”   “The Scarlet Letter.™” “Tainted.”)

Taint mode is a feature of Perl that attempts to keep track of ... not only the value of a particular variable, but also its tainted-ness.   And to warn you, more-or-less automatically, when you might be using a “tainted” value unwittingly.

It’s a feature of Perl that is, shall we say, “a very pleasant surprise” (in the right situations).   A little-bit unexpected, perhaps, and therefore probably a surprise ... but tremendously useful once you understand it.