#!/usr/local/bin/perl5.9.3

use assertions '1'; # assrt. are always active

use strict;
use warnings;

sub assert (&@) :assertion {
  my $sub = shift;
  &{$sub}() or die "assertion failed: @_"
}

sub my_sqrt {
  my $n = shift;
  assert { $n >=0 } "argument has to be a positive number";
  return sqrt($n);
}

for my $n (0.3, 0, -4, 7, 9) {
  printf "sqrt(%f) = %f\n", $n, my_sqrt($n);
}