The docs for WWW::Mechanize::Plugin::JavaScript say
Version 0.010 (alpha)

THIS MODULE IS DEPRECATED. Please use WWW::Scripter::Plugin::JavaScript instead.

Aside from that, it appears neither module is available via PPM, so you will have to follow the standard instructions in the README

In reply to Re: How to install WWW::Mechanize::Plugin::JavaScript on windows by Anonymous Monk
in thread How to install WWW::Mechanize::Plugin::JavaScript on windows by sanPerl

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