++ed by:
Author image Вячеслав Тихановский
and 4 contributors


Devel::Mutator - Mutation testing for Perl


    mutator mutate lib/MyModule.pm
    mutator test


Devel::Mutator is a mutation testing toolkit for Perl. Mutation testing is changing the working program in different ways and checking that the test suite fails and thus detecting the bad testing.

How it works

First we generate mutated code. For example every occurance of = is replaced by !=. All the mutants are collected in the mutants/ directory. Then we run the tests replacing original code by the mutant. If the test suite does not fail when the code is changed it is reported with a diff output, which helps to see the problem.

    (10/18) ./mutants/7021082cc1c0afbe9322f60a9b5e5d5f/lib/Input/Validator/Field.pm ... not ok
    --- ./mutants/7021082cc1c0afbe9322f60a9b5e5d5f/lib/Input/Validator/Field.pm Sat Nov  1 11:27:00 2014
    +++ lib/Input/Validator/Field.pm.bak    Sun May 18 21:50:14 2014
    @@ -14,7 +14,7 @@
         my $self = shift;

         $self->{constraints} ||= [];
    -    $self->{messages}    //= {};
    +    $self->{messages}    ||= {};

         $self->{trim} = 1 unless defined $self->{trim};

Here we can see that the test suite does not check the need for //.


The original code is replaced by the mutants, so make sure it is under a VCS if something bad happens. This is the easiest and the 100% working way. Maybe this will be changed in the future when a better way is found.

Mutation testing drawbacks

There are several drawbacks.

The equivalent program

The equivalent program can be produced thus failing the test. There is no solution to that for now.

Infinite loops

Infinite loops can be created easily. The solution is to run the test suite limited in time through a timeout option, which is 10s by default. After 10s of running, the test suite is killed and n/a timeout is reported.



Alexandr Ciornii (chorny)

Tim Teasdale (hooverbag)

Patricio Valle (pvallev)


Viacheslav Tykhanovskyi, <viacheslav.t@gmail.com>


Copyright (C) 2015, Viacheslav Tykhanovskyi

This program is free software, you can redistribute it and/or modify it under the terms of the Artistic License version 2.0.

This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose.