Test::Subroutines - Standalone execution of Perl program subroutines


version 1.113350


You have a (possibly ancient) Perl program for which you'd like to write some unit tests. The program code cannot be modified to accommodate this, and you want to test subroutines but not actually run the program. This module permits running of the program subroutines standalone, and in relative safety.


 use Test::Subroutines; # exports load_subs
 # set up any globals to match those in the Perl program
 my $global = 'foo';
 load_subs( $perl_program_file );
 # subs from $perl_program_file are now available for calling directly
 # OR
 load_subs( $perl_program_file, $namespace );
 # subs from $perl_program_file are now available for calling in $namespace


You'll need to set-up any environment the subroutines may need, such as global lexical variables, and also be aware that side effects from the subroutines will still occur (e.g. database updates).

Load the module like so:

 use Test::Subroutines;

Then use load_subs() to inspect your program and make available the subroutines within it. Let's say your program is /usr/bin/myperlapp. The simplest call exports the program's subroutines into your own namespace so you can call them directly. Note use of the & subroutine sigil which is required:

 load_subs( '/usr/bin/myperlapp' );
 # and then...
 $retval = &myperlapp_sub($a,$b);

If the subroutines happen to use global lexicals in the program, then you do need to set these up in your own namespace, otherwise load_subs() will die with an error message. Note that they must be lexicals - i.e. using my.

If you don't want your own namespace polluted, then load the subroutines into another namespace:

 load_subs( '/usr/bin/myperlapp', 'MyTestNamespace' );
 # and then...
 $retval = &MyTestNamespace::myperlapp_sub($a,$b);

Note that this namespace must not be nested, in other words it cannot contain the :: characters. This is a simple limitation which could be patched.

Catching exit() and other such calls

There's the potential for a subroutine to call exit(), which would seriously cramp the style of your unit tests. All is not lost, as by default this module installs a hook which turns exit() into die(), and in turn die() can be caught by an eval as part of your test. You can override the hook by passing a HASH reference to load_subs, like so:

 load_subs( '/usr/bin/myperlapp', {
     exit => sub { $_[0] ||= 0; die "caught exit($_[0])\n" }
 } );

In fact the example above is the default hook - it dies with that message. Pass a subroutine reference as shown above and you can get exit() to do whatever you like. With the default hook, you might have this in your tests:

 # unit test
 eval { &sub_which_exits($a,$b) };
 is( $@, 'caught exit(0)', 'subroutine exit!' );

Finally, a similar facility to that described here for overriding exit() is available for the system() builtin as well. The default hook for system() is a noop though - it just allows the call to system() to go ahead.


  • You have to call the subroutines with leading & to placate strict mode.

  • Warnings of category closure are disabled in your loaded program.

  • You have to create any required global lexicals in your own namespace.


Some folks on IRC were particularly helpful with suggestions: batman, mst and tomboh. Thanks, guys!


Oliver Gorwits <>


This software is copyright (c) 2011 by University of Oxford.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.