To use this merge module, your Perl::Dist::WiX subclass has to use these parameters to new():

...
msm_to_use           => '<URL location of [% dist.output_base_filename %].msm>',
msm_zip              => '<URL location of [% dist.output_base_filename %].zip>',
msm_code             => '[% dist.msm_package_id %]',
[% IF dist.relocatable %]
fileid_perl          => '[% dist.fileid_perl %]',
fileid_relocation_pl => '[% dist.fileid_relocation_pl %]',
[% END %]...

In addition, your 'tasklist' parameter has to use 'initialize_using_msm' as its second entry (right after 'final_initialization'.)