the OP only wants two processes in parallel which is too obviously not a case where undef can occur. 
$ cat cantfork.pl 
#!/usr/bin/perl
use strict;
use warnings;

my $child = fork;
die "Well that's odd - the fork failed" unless defined $child;

if ($child) {
    print "Parent! Child is $child\n";
} else {
    print "Child!\n";
}

sleep 1;
exit;
$ ./cantfork.pl 
Parent! Child is 17198
Child!
$ ulimit -u 9
$ ./cantfork.pl 
Well that's odd - the fork failed at ./cantfork.pl line 6.

[download]

Always code defensively, especially when using system calls as they can fail at any time for any reason.

In reply to Re^12: Doing every X seconds by hippo
in thread Doing every X seconds by dideod.yang

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