$ 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.