system("ls -l > ls.out");  # you gonna have to wait
system("ls -l > ls.out &"); # the command goes into the
                            # background 

##</code><code>##

my $pid = fork;
die "Fork failed me!\n" if !defined($pid);
if ( $pid == 0 ) {
   # I'm the child!
   exec("ls -l > ls.out");
}
# I'm the parent!
sleep 5; #or whatever