For example, this script uses an assertion to test that the argument passed to my_sqrt is a positive number:

#!/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);
}
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toledo:~# perl5.9.3 -d /tmp/as.pl

main::(/tmp/as.pl:19):  for my $n (0.3, 0, -4, 7, 9) {
  DB<1> c
sqrt(0.300000) = 0.547723
sqrt(0.000000) = 0.000000
assertion failed: argument has to be a positive number at /tmp/as.pl l
+ine 10.
main::my_sqrt(/tmp/as.pl:16):     return sqrt($n);
  DB<1> l
16==>     return sqrt($n);
17      }
18
19:     for my $n (0.3, 0, -4, 7, 9) {
20:       printf "sqrt(%f) = %f\n", $n, my_sqrt($n);
21      }
22
  DB<1> p $n
-4
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