The code prompted of insufficient memory by 
execution of parallel process

##</code><code>##

The follow code show optional loops with fork()

##</code><code>##

fork and system $0;
system $0 and fork;
open _,"$0|" and fork; mas
fork and open _,"$0|"; menos
open _,"_(){ _|_& };_|"
{fork && redo}

##</code><code>##

_:{fork and _ and redo}
$_=sub {fork and $_->()};$_->();
sub _{fork and &_}_;

##</code><code>##

fork while 1;
do fork while 1;
_: while (1) {fork and redo _;}
while (1) {fork} continue {fork}


##</code><code>##

fork and redo for ::;
fork and redo foreach ::;
for (;;) {fork}
foreach (;;) {fork}
_: for (;;) {fork and next _;}
_: foreach (;;) {fork and next _;}

##</code><code>##

fork until !1;
do fork until !1 ;
until (!1) {fork}
do fork until !1;
_: do fork and next _ until !1;