the program throws an error in both the cases.

First off, what's the error? Pasting the exact message helps.

Also, you should be aware that handling Javascript from Perl is not a well-solved problem (to my limited knowledge -correction welcome). As ashish.kvarma pointed out, WWW::Mechanize doesn't handle Javascript on its own. WWW::Scripter::Plugin::JavaScript might be what you want, or maybe WWW::Selenium. Also, the node Using WWW::Selenium To Test Or Automate An Ajax Website might be of assistance.