Thanks for elaborating on utf8 and suggesting on a way forward with my questions. As is frequently the case, the monastery has another thread going to discuss the same issue: Can we write multi-threaded scripts ?. I looked at the reference that localshop posted and found code that uses perl's native language to thread. I would prefer not to have to use a module just for this little task. I must say, however that I fall short on my first attempt in some ways. Output then source:

$ ./2.fork.pl 
this is task 1 with pid 0
Waiting for child processes..
this is task 2 with pid 0
this is task 3 with pid 0
44
bing's translation is 
unctuous hypocrisy flowing from the tube

елейным лицемерие течет из трубки

 English -> Русский 
Child with PID=23085 finished..
43
42
41
slept 5
40
39
38
ERROR Oops! Something went wrong and I can't translate it for you :(
yandex's translation is 
Child with PID=23086 finished..
37
36
slept 10
35
google's translation is 
unctuous hypocrisy flowing from the tube

неприступное лицемерие, вытекающее из трубки
(nepristupnoye litsemeriye, vytekayushcheye iz trubki)

Translations of unctuous hypocrisy flowing from the tube
 English -> Русский 

unctuous hypocrisy flowing from the tube
    неприступное лицемерие, вытекающее из трубки, елейное лицемерие течет из трубки
Child with PID=23087 finished..
Done.
$ cat 2.fork.pl 
#!/usr/bin/perl -w
use 5.011;

my $string = "unctuous hypocrisy flowing from the tube";

my $pid1 = fork();
if ( $pid1 == 0 ) {    # Task 1
    say "this is task 1 with pid $pid1";
    my $command = "trans :ru -e bing \"$string\" >1.bing.txt"; 
    system("$command");
    say "bing's translation is ";
    system("cat 1.bing.txt");
    exit 0;
}

my $pid2 = fork();
if ( $pid2 == 0 ) {    # Task 2
    say "this is task 2 with pid $pid2";
    sleep 5;
    say "slept 5";
    system("trans :ru -e yandex \"$string\" >1.yandex.txt");
    say "yandex's translation is ";
    system("cat 1.yandex.txt");
    exit 0;
}

my $pid3 = fork();
if ( $pid3 == 0 ) {    # Task 3
    say "this is task 3 with pid $pid3";
    sleep 10;
    say "slept 10";
    system("trans :ru -e google \"$string\" >1.google.txt");
    say "google's translation is ";
    system("cat 1.google.txt");
    exit 0;
}

say "Waiting for child processes..";
my $counter = 45;
local $SIG{ALRM} = sub {
    say --$counter;
    alarm 1;
};
alarm 1;

while ((my $pid = wait) != -1) {
    say "Child with PID=$pid finished..";
}

alarm 0;
say "Done.";
__END__

Even with all the say statements dropped in, I find execution hard to follow. I'm baffled that pid's are zero within a block, but not so when they finish. What alarm truly does here is unclear. Finally, there is no code to kill pid's when the timer reaches zero.

I'm also fishing for code that would do this within the following loop:

  print "Get other translations(y/n)?: ";
  my $prompt = <STDIN>;
  chomp $prompt;
  if ( $prompt eq ( "y" | "Y" ) ) {
    my @translators = qw /yandex bing/;
    for my $remote (@translators) {
      my $trans_munge = path( $vars{translations}, "$remote." . $munge
+ );

      ## use trans shell
      say "getting translation from $remote";
      system("trans :$lang -e $remote file://$in_path >$trans_munge");
    }
[download]

Again, thanks for the very helpful comments.

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]