If I've got this right, your code is already prevented from being a fork bomb. Your script forks off a single child, then waits for that child to terminate, before forking off the next child. :) I don't think that's what you were looking for, though...

According to the docs for wait and waitpid, the status of the child process is returned in $?. Thus, your child can exit with a status code to indicate whether it succeeded, and the parent can use that to determine whether or not to increment $avail.

Here's a quick example based on some code from perlipc. (Please note that prior to 5.6.0 the relevant code sample in perlipc omitted the > 0 in the while condition.)

#!/usr/local/bin/perl -w

use strict;

use POSIX ":sys_wait_h";

my $avail;

sub REAPER {
    my $child;
    while ($child = waitpid(-1,&WNOHANG()) > 0) {
        $avail += ! $?;  # 0 means success, non-zero means failure
    }
    $SIG{CHLD} = \&REAPER;
}

$SIG{CHLD} = \&REAPER;

for (0..2,0..2) {
    if (fork) {
    } else {
        exit $_;
    }
}

print "$avail\n";
[download]