As a side note, mechanize seems overkill for that task.

While not strictly core, LWP::Simple should be on most systems as dependency of the cpan client (well IMHO I didn't check yet°)

So just get("url") the $content and then open the reference \$content to a filehandle.

°) I'm not sure anymore about the most basic "fetch from Web" mechanism of cpan-client, but LWP was one of the options my system used...