It's seems to me it's a feature actually and not a bug. Quote from the Changes for version 1.51_03: 
$mech->follow_link() did not complain if a link could not be found,
even with autocheck on.  Now it does.  Thanks, Flavio Poletti.

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In reply to Re: WWW::Mechanize follow_link() crashes if no link is found by arkturuz
in thread WWW::Mechanize follow_link() crashes if no link is found by venkatesan_G02

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