The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.


Devel::Cover - Code coverage metrics for Perl


To get coverage for an uninstalled module:

 cover -test


 cover -delete
 HARNESS_PERL_SWITCHES=-MDevel::Cover make test

To get coverage for an uninstalled module which uses Module::Build (0.26 or later):

 ./Build testcover

If the module does not use the t/*.t framework:

 PERL5OPT=-MDevel::Cover make test

If you want to get coverage for a program:

 perl -MDevel::Cover yourprog args

 perl -MDevel::Cover=-db,cover_db,-coverage,statement,time yourprog args


This module provides code coverage metrics for Perl. Code coverage metrics describe how thoroughly tests exercise code. By using Devel::Cover you can discover areas of code not exercised by your tests and determine which tests to create to increase coverage. Code coverage can be considered as an indirect measure of quality.

I consider this software to have an alpha status. By that I mean that I reserve the right to alter the interface in a backwards incompatible manner without incrementing the major version number. I specifically do not mean that this software is full of bugs or missing key features. Although I'm making no guarantees on that front either. In short, if you are looking for code coverage software for Perl, you have probably come to the end of your search. For more of my opinions on this subject, see

Code coverage data are collected using a pluggable runops function which counts how many times each op is executed. These data are then mapped back to reality using the B compiler modules. There is also a statement profiling facility which needs a better backend to be really useful. This release also includes an experimental mode which replaces ops instead of using a pluggable runops function. This provides a nice speed increase, but needs better testing before it becomes the default. You probably don't care about any of this.

The cover program can be used to generate coverage reports.

Statement, branch, condition, subroutine, pod and time coverage information is reported. Statement coverage data should be reasonable, although there may be some statements which are not reported. Branch and condition coverage data should be mostly accurate too, although not always what one might initially expect. Subroutine coverage should be as accurate as statement coverage. Pod coverage comes from Pod::Coverage. If Pod::Coverage::CountParents is available it will be used instead. Coverage data for path coverage are not yet collected.

The gcov2perl program can be used to convert gcov files to Devel::Cover databases.

You may find that the results don't match your expectations. I would imagine that at least one of them is wrong.

The most appropriate mailing list on which to discuss this module would be perl-qa. Discussion has migrated there from perl-qa-metrics which is now defunct. See

The Devel::Cover repository can be found at


  • Perl 5.6.1 or greater. Perl 5.8.2 or greater is recommended.

    Perl 5.7.0 is unsupported. Perl 5.8.2 or greater is recommended. Whilst Perl 5.6 should mostly work you will probably miss out on coverage information which would be available using a more modern version and will likely run into bugs in perl. Perl 5.8.0 will give slightly different results to more recent versions due to changes in the op tree.

  • The ability to compile XS extensions.

    This means a working compiler and make program at least.

  • Storable and Digest::MD5

    Both are in the core in Perl 5.8.0 and above.

  • Template and PPI::HTML or Perl::Tidy

    if you want syntax highlighted HTML reports.

  • Pod::Coverage

    if you want Pod coverage.

  • Test::More

    in order to run the tests

  • Test::Warn

    in order to run some of the tests

  • Test::Differences

    if the tests fail and you would like nice output telling you why.


 -blib               - "use blib" and ignore files matching \bt/ (default true
                       iff blib directory exists).
 -coverage criterion - Turn on coverage for the specified criterion.  Criteria
                       include statement, branch, condition, path, subroutine,
                       pod, time, all and none (default all available).
 -db cover_db        - Store results in coverage db (default ./cover_db).
 -dir path           - Directory in which coverage will be collected (default
 -ignore RE          - Set REs of files to ignore (default "/Devel/Cover\b").
 +ignore RE          - Append to REs of files to ignore.
 -inc path           - Set prefixes of files to ignore (default @INC).
 +inc path           - Append to prefixes of files to ignore.
 -merge val          - Merge databases, for multiple test benches (default on).
 -select RE          - Set REs of files to select (default none).
 +select RE          - Append to REs of files to select.
 -silent val         - Don't print informational messages (default off).
 -subs_only val      - Only cover code in subroutine bodies (default off).
 -replace_ops val    - Use op replacing rather than runops (default on).
 -summary val        - Print summary information iff val is true (default on).

More on Coverage Options

You can specify options to some coverage criteria. At the moment only pod coverage takes any options. These are the parameters which are passed into the Pod::Coverage constructor. The extra options are separated by dashes, and you may specify as many as you wish. For example, to specify that all subroutines containing xx are private, call Devel::Cover with the option -coverage,pod-also_private-xx.


You may select which files you want covered using the select, ignore and inc options. The system works as follows:

Any file matching a RE given as a select option is selected.

Otherwise, any file matching a RE given as an ignore option is ignored.

Otherwise, any file in one of the inc directories is ignored.

Otherwise the file is selected.

You may add to the REs to select by using +select, or you may reset the selections using -select. The same principle applies to the REs to ignore.

The inc directories are initially populated with the contents of the @INC array at the time Devel::Cover was built. You may reset these directories using -inc, or add to them using +inc.

Although these options take regular expressions, you should not enclose the RE within // or any other quoting characters.


The -silent option is turned on when Devel::Cover is invoked via $HARNESS_PERL_SWITCHES or $PERL5OPT. Devel::Cover tries to do the right thing when $MOD_PERL is set. $DEVEL_COVER_OPTIONS is appended to any options passed into Devel::Cover.

When running Devel::Cover's own test suite, $DEVEL_COVER_DEBUG turns on debugging information, $DEVEL_COVER_GOLDEN_VERSION overrides Devel::Cover's own idea of which golden results it should test against, and $DEVEL_COVER_NO_COVERAGE runs the tests without collecting coverage.


Some code and ideas cribbed from:





There are things that Devel::Cover can't cover.

Absence of shared dependencies

Perl keeps track of which modules have been loaded (to avoid reloading them). Because of this, it isn't possible to get coverage for a path where a runtime import fails if the module being imported is one that Devel::Cover uses internally. For example, suppose your program has this function:

 sub foo {
     eval { require Storable };
     if ($@) {
         carp "Can't find Storable";
     # ...

You might write a test for the failure mode as

 BEGIN { @INC = () }
 # check for error message

Because Devel::Cover uses Storable internally, the import will succeed (and the test will fail) under a coverage run.

Modules used by Devel::Cover while gathering coverage:

  • B

  • B::Debug

  • B::Deparse

  • Carp

  • Cwd

  • Digest::MD5

  • File::Path

  • File::Spec

  • Storable


By adding use Devel::Cover; to your mod_perl startup script, you should be able to collect coverage information when running under mod_perl. You can also add any options you need at this point. I would suggest adding this as early as possible in your startup script in order to collect as much coverage information as possible.

Redefined subroutines

If you redefine a subroutine you may find that the original subroutine is not reported on. This is because I haven't yet found a way to locate the original CV. Hints, tips or patches to resolve this will be gladly accepted.


Almost certainly.

See the BUGS file. And the TODO file.


Version 0.73 - 2nd October 2010


Copyright 2001-2010, Paul Johnson (

This software is free. It is licensed under the same terms as Perl itself.

The latest version of this software should be available from my homepage: