In extreme cases you might need to tinker with what Perl has loaded in memory.

In less extrme cases, you might wonder, why changing a module, even rendering it syntactically incorrect does not have any impact on your script. Is it even loading that copy of the module?

Perl provides a variable called %INC, it is slightly related to @INC but not to be confused with it. %INC contains information about modules that have been loaded into memory by the currently running process.

The most simple way to see it, is by using Data::Dumper with the following one-liner:

perl -MData::Dumper -e 'print Dumper \%INC'.

(On Windows, you will want to use double-quotes instead of single-quotes around the expression.) 

$VAR1 = {
  'strict.pm' => '/opt/perl-5.20.1/lib/5.20.1/strict.pm',
  'overload.pm' => '/opt/perl-5.20.1/lib/5.20.1/overload.pm',
  'Carp.pm' => '/opt/perl-5.20.1/lib/5.20.1/Carp.pm',
  'constant.pm' => '/opt/perl-5.20.1/lib/5.20.1/constant.pm',
  'warnings/register.pm' => '/opt/perl-5.20.1/lib/5.20.1/warnings/register.pm',
  'XSLoader.pm' => '/opt/perl-5.20.1/lib/5.20.1/XSLoader.pm',
  'warnings.pm' => '/opt/perl-5.20.1/lib/5.20.1/warnings.pm',
  'bytes.pm' => '/opt/perl-5.20.1/lib/5.20.1/bytes.pm',
  'Exporter.pm' => '/opt/perl-5.20.1/lib/5.20.1/Exporter.pm',
  'overloading.pm' => '/opt/perl-5.20.1/lib/5.20.1/overloading.pm',
  'Data/Dumper.pm' => '/opt/perl-5.20.1/lib/5.20.1/darwin-thread-multi-2level/Data/Dumper.pm',
  'vars.pm' => '/opt/perl-5.20.1/lib/5.20.1/vars.pm'
};

The keys are the module names mapped to the file names that are searched for in @INC, the values are the full pathes to the files that were actually loaded.

You might wonder, which one of these modules are loaded by perl by default. For that we need to go back one step and print %INC without loading Data::Dumper.

In perl -e 'print join "\n", %INC; print "\n"' we join together all the keys and values of %INC with newlines. But it prints nothing. That is, perl itself does not load any module.

We can then try to load some of the pragma: 

$ perl -e 'use strict; print join "\n", %INC; print "\n"'
strict.pm
/opt/perl-5.20.1/lib/5.20.1/strict.pm


$ perl -e 'use warnings; print join "\n", %INC; print "\n"'
warnings.pm
/opt/perl-5.20.1/lib/5.20.1/warnings.pm

Each one loads itself and nothing else.

Interesting side note: providing the -E flag instead of the -e flag will automatically load the feature pragma. This might not be surprising if you have seen that -E enables all optional features. (See in perlrun.) 

$ perl -E 'print join "\n", %INC; print "\n"'
feature.pm
/opt/perl-5.20.1/lib/5.20.1/feature.pm

For some nicer layout, and without loading Data::Dumper, you might want to try the following code:

For example to see what Moo loads:

perl -e 'use Moo; printf qq{%30s %s\n}, $_, $INC{$_} for sort keys %INC;'

(Windows user will want to replace the single-quotes by double-quotes.)

The output looks like this: 

                       Carp.pm /opt/perl-5.20.1/lib/5.20.1/Carp.pm
                     Config.pm /opt/perl-5.20.1/lib/5.20.1/darwin-thread-multi-2level/Config.pm
    Devel/GlobalDestruction.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Devel/GlobalDestruction.pm
                   Exporter.pm /opt/perl-5.20.1/lib/5.20.1/Exporter.pm
                Import/Into.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Import/Into.pm
                  List/Util.pm /opt/perl-5.20.1/lib/5.20.1/darwin-thread-multi-2level/List/Util.pm
             Module/Runtime.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Module/Runtime.pm
                        Moo.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo.pm
   Moo/HandleMoose/_TypeMap.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo/HandleMoose/_TypeMap.pm
                 Moo/Object.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo/Object.pm
                 Moo/_Utils.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo/_Utils.pm
                   Moo/_mro.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo/_mro.pm
             Moo/sification.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Moo/sification.pm
                Scalar/Util.pm /opt/perl-5.20.1/lib/5.20.1/darwin-thread-multi-2level/Scalar/Util.pm
   Sub/Exporter/Progressive.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/Sub/Exporter/Progressive.pm
                   Sub/Name.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/darwin-thread-multi-2level/Sub/Name.pm
                   XSLoader.pm /opt/perl-5.20.1/lib/5.20.1/XSLoader.pm
                       base.pm /opt/perl-5.20.1/lib/5.20.1/base.pm
                   constant.pm /opt/perl-5.20.1/lib/5.20.1/constant.pm
                       main.pm -e
                        mro.pm /opt/perl-5.20.1/lib/5.20.1/darwin-thread-multi-2level/mro.pm
                     strict.pm /opt/perl-5.20.1/lib/5.20.1/strict.pm
                 strictures.pm /opt/perl-5.20.1/lib/site_perl/5.20.1/strictures.pm
                       vars.pm /opt/perl-5.20.1/lib/5.20.1/vars.pm
                   warnings.pm /opt/perl-5.20.1/lib/5.20.1/warnings.pm
          warnings/register.pm /opt/perl-5.20.1/lib/5.20.1/warnings/register.pm

This can make it easy to see if a module that you have changed has been loaded, and from which location in the filesystem. Maybe @INC is not set up correctly and you are using the installed version of the module instead of the development version? In that case you might want to see how to add a relative directory to @INC or how to change @INC to find modules in non-standard locations.