the new assertion subs, available on blead perl and on 5.10 in the future, act as conditional breakpoints under the debugger.

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);
}
[download]

then running it under the debugger:

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
[download]