My personal experience on Linux and Windows is that pretty bad with WWW::Mechanize as it tends to leak from my pov.
How many pages are you looking at?
I am downloading between 1500-2000 web pages and the leak was somewhere inside WWW::Mechanize as I did not have the leak after changing the URL class. With the simple UA class there was no leak and I gained a lot of speed by running several UAs in parallel.