use Carp qw(cluck);
my $cluck = 0;

sub _f_tail_rec {
    if($_[0] == 0) {
        cluck "Goto optimized stack trace" if $cluck;
        return $_[1]
    }   
    $_[1] *= $_[0];
    $_[0]--; 
    goto &_f_tail_rec;
}   
sub f_tail_rec { @_ = (@_, 1); goto &_f_tail_rec;}

my $n = 10;
print "f_tail_rec($n) = ".f_tail_rec($n),"\n";