I'm baffled that pid's are zero within a block, but not so when they finish.

fork causes the execution of a process to split into two different processes, sharing the initial memory layout but not the following changes to the variables. To help the program discern which of the two it has become, the newly produced ("child") process receives 0 as the return value of fork() and the old one ("parent") gets returned the PID of the new process. This is why you had to put an exit 0; at the end of the if ($pid == 0) block: otherwise both the parent and the child would continue executing the same parts of the program, causing a lot of confusion.

wait returns the real (former) PIDs of the freshly terminated child processes. If the child wanted for some reason to know its PID, it could have used the $$ variable.

This example might help:

if (fork() == 0) {
 system("sh", "-c", "sleep 3 # getting yandex translation");
 exit(0);
}
if (fork() == 0) {
 system("sh", "-c", "sleep 2; # getting google translation");
 exit(0);
}

system("pstree -Apal $$"); # pstree gets the PID of the parent process
+ as its argument

while((my $pid = wait()) != -1) {
 say "$pid terminated"
}
__END__
perl,29173 test.pl
  |-perl,29174 test.pl
  |   `-sh,29177 -c sleep 3 # getting yandex translation
  |       `-sleep,29179 3
  |-perl,29175 test.pl
  |   `-sh,29178 -c sleep 2; # getting google translation
  `-pstree,29176 -Apal 29173
29175 terminated
29174 terminated
[download]

pstree command draws all child processes of the main Perl process (including itself) with their command line arguments. The Perl processes having the same command line arguments but different PIDs are the copies created by fork().

What alarm truly does here is unclear. Finally, there is no code to kill pid's when the timer reaches zero.

alarm arranges for a SIGALRM signal to be delivered to the calling process in the number of seconds specified as its argument. It behaves as if something else kills the process which had called alarm and forgot to disarm it. The good part is that one can use $SIG{ALRM} to trap the signal and do something meaningful instead of dying, but in your case the work is already being done in a child process, and we can let it be killed:

my $start = time;

if (fork() == 0) {
 # arm the alarm clock
 alarm(10);
 # create a child process that sleeps
 system("sleep 365d");
 exit(0);
}

system("pstree -Apal $$");

while((my $pid = wait()) != -1) {
 say "$pid terminated"
}

say time-$start, " seconds elapsed instead of a year"
__END__
perl,29620
  |-perl,29621
  |   `-sleep,29623 365d
  `-pstree,29622 -Apal 29620
29621 terminated
10 seconds elapsed instead of a year
[download]

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