perldoc -f kill claims that kill with a signal 0 does nothing. 0 * anything is 0, so if $_[0] is not tainted, nothing happens.

However, if $_[0] is tainted, then the code will die with "Insecure dependency in kill while running with -T switch at -e line 1, <STDIN> l ine 1.". So if $_[0] is tainted, $@ matches. If $_[0] is not tainted, no match.