Perl is huge. It's used in a lot of diverse places. Web scraping is huge. It's done all the time. Javascript is huge. It's ubiquitous in the context of the web. WWW::Mechanize doesn't directly handle Javascript. The number of people who have encountered this issue is also huge. It's a highly searchable topic that has been answered many times -- a FAQ (which might explain the beating you're taking in votes right now). And it's also why the POD for WWW::Mechanize has the following:

Please note that Mech does NOT support JavaScript, you need additional software for that. Please check "JavaScript" in WWW::Mechanize::FAQ for more.

In the WWW::Mechanize::FAQ you will find a section titled "JavaScript". There's a good explanation there on how it may be possible to simply provide the proper get/post requests instead of relying on JavaScript parsing to deal with the website. But it also has a subcategory, "Which modules work like Mechanize and have JavaScript support?". You'll find some good suggestions there on how to proceed.