$ perl -E'
if (my $pid = fork) {
 say "My pid is $$, child is $pid";
 local $SIG{USR1} = sub { say "parent"; exit 0 };
 sleep 1 while 1
} else {
 local $SIG{USR1} = sub { say "child"; exit 0 };
 sleep 1 while 1
}' &
[1] 19681
My pid is 19681, child is 19683
$ kill -USR1 19683
child
$ kill -USR1 19681
parent
[1]+  Done                    perl -E'if (my $pid = fork) { say "My pid is $$, child is $pid"; local $SIG{USR1} = sub { say "parent"; exit 0 }; sleep 1 while 1 } else { local $SIG{USR1} = sub { say "child"; exit 0 }; sleep 1 while 1}