An alternative (best is to use modules, it saves you from deadlock pitfalls) is using pipes:

perlipc#Safe-Pipe-Opens

The problem with pipes is that you can kill your child, or the child hangs or takes too long, without the parent knowing, as it is "reading". So you need an alarm ($SIG{'ALRM'}) in your parent. A way I do it (so the parent stays responsive)

 #!/usr/bin/perl 
 use strict;
 use warnings;
 
my $parent_pid = $$;
my $child_pid = 0;
my @child_output;

my $read_from_child = 0;
my $max_timeout = 60;
my $timeout;
    
print "Parent is $parent_pid\n";

my $pid = open(KID_TO_READ, "-|");
defined($pid) || die "can't fork: $!";
#local $SIG{CHLD};
if ($pid) {             # parent
    local $SIG{'CHLD'} = sub { print "dead child\n"; $timeout=0 };
    local $SIG{'USR1'} = sub { ++$read_from_child };
    print "reading kid...\n";
    $timeout = $max_timeout;
    while($timeout >0){
        print "parent $parent_pid timeout=$timeout read_from_child=$re
+ad_from_child - child_pid=$child_pid\n";
        if($read_from_child>0){
            --$read_from_child;
            $_=<KID_TO_READ>;
            print "parent $$ just read: $_";
            unless( $child_pid>0 ){
                chomp($child_pid = $_);
            }else{
                push @child_output, $_;
            }            
        }else{
            sleep 1;
            --$timeout;
        }
    }
    close(KID_TO_READ);
} else {                # child
    print $$ . "\n";    # send child pid
    kill('USR1', $parent_pid); # warn parent there is data
    print STDERR "child $$ is running\n";  # print to screen (DEBUG)
    my $program = "/bin/sleep";
    my @args = qw(3);
    my $ret = system($program, @args);
    print "returncode is $ret\n"; # is $?
    print STDERR "returncode is $ret\n"; # print to screen (DEBUG)
    kill('USR1', $parent_pid); # warn parent there is data
    exit 0;
}

print "\nChild $child_pid is done, we now have:\n@child_output\n";
[download]

which prints something like:

Parent is 5323
reading kid...
parent 5323 timeout=60 read_from_child=0 - child_pid=0
child 5324 is running
parent 5323 timeout=59 read_from_child=1 - child_pid=0
parent 5323 just read: 5324
parent 5323 timeout=59 read_from_child=0 - child_pid=5324
parent 5323 timeout=58 read_from_child=0 - child_pid=5324
parent 5323 timeout=57 read_from_child=0 - child_pid=5324
parent 5323 timeout=56 read_from_child=0 - child_pid=5324
returncode is 0
parent 5323 timeout=55 read_from_child=1 - child_pid=5324
parent 5323 just read: returncode is 0
parent 5323 timeout=55 read_from_child=0 - child_pid=5324
dead child

Child 5324 is done, we now have:
returncode is 0
[download]

See also:

Re: How to make parent wait for all the child processes.

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