Interestingly, the kill procedure in Perl explicitely checks for non-numbers, which is unusual. Perl as a language would just cast undef to zero in a numeric context[1], and 'kill $signal, 0' would go on killing every process in the process group.

  $ perl -we 'kill 9, undef'
  Can't kill a non-numeric process ID at -e line 1.

[1] unless you "use warnings FATAL => 'uninitialized';"