re - Perl pragma to alter regular expression behaviour
($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here use re "taint"; ($x) = ($^X =~ /^(.*)$/s); # $x _is_ tainted here
(We use $^X in these examples because it's tainted by default.)
use re 'taint' is in effect, and a tainted string is the target of a regex, the regex memories (or values returned by the m// operator in list context) are tainted.
This feature is useful when regex operations on tainted data aren't meant to extract safe substrings, but to perform other transformations.