sub fib
{
    my $n = shift;
    if ($n < 2) {
        return $n;
    } else {
        return fib($n-1) + fib($n-2);
    }
}
__END__
$ time perl -le 'BEGIN { sub DB::DB {}; $^P=0x303 }; do "/tmp/fib.pl"; print fib(28)'
317811

real	0m0.949s
user	0m0.941s
sys	0m0.006s
$ time perl -le 'do "/tmp/fib.pl"; print fib(28)'
317811

real	0m0.924s
user	0m0.918s
sys	0m0.005s
$ time perl -le 'BEGIN { sub DB::DB {}; $^P=0xfff }; do "/tmp/fib.pl"; print fib(28)'
317811

real	0m1.719s
user	0m1.712s
sys	0m0.006s
$ time perl -le 'BEGIN { sub DB::DB {}; $^P=0x303; $DB::trace=1 }; do "/tmp/fib.pl"; print fib(28)'
317811

real	0m1.557s
user	0m1.548s
sys	0m0.006s