My guess is that you have one or more logic errors in the program.    I'm also not sure why you're using @ARGV -- is this just to control how many processes get started?  If so, that's okay, but it's a little misleading that you call the loop variable $process.

You may also be expecting that get_pidof is returning the process id only of any running fork.pl programs, but you may have others -- for example, if you're editing fork.pl in the background, then get_pid can match it instead.  So first of all, try passing a second argument to get_pidof:

my @pids = $get_pidof('fork.pl', 1);
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which should cause it to find processes which exactly match fork.pl.

Next of all, why are you decrementing $childcnt when the return from the wait is -1?  According to perldoc -f wait:

       wait    Behaves like the wait(2) system call on your system: it
+ waits
               for a child process to terminate and returns the pid of
+ the
               deceased process, or "-1" if there are no child process
+es.  The
               status is returned in $?.  Note that a return value of 
+"-1"
               could mean that child processes are being automatically
+ reaped,
               as described in perlipc.
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Which means you probably want to decrement only when you get a value greater than zero for $completed_pid.  Additionally, you are executing a blocking wait, so the wait will block until it succeeds, or returns -1 for "no more children".  If you want to make it non-blocking, try something like this:

while (1) {
    my $completed_pid = waitpid(-1, WNOHANG);
    if ($completed_pid == -1) {
        # No more children -- exit loop
        last;
    }
    if ($completed_pid > 0) {
        print "<parent>: $completed_pid is done; $childcnt running\n";
            $childcnt--;
        sleep 1;
    }
}
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You'll also need use POSIX ":sys_wait_h"; at the top of the program.  You can read more about non-blocking waits with perldoc -f waitpid.

Update:  I discovered another possible reason why you're getting a negative $childcount when you use get_pidof() ... it's because get_pidof is itself spawning a process which is a child of fork.pl.  This causes your program to wait for it and it decrements $childcount one too many times.