What's weird in that observation is that url_abs should give the correct link, even if your base element was ignored.

use 5.020;
use strict;
use URI::URL;
my $uri = URI::URL->new('page2','http://domain.com/page');
say "base is", $uri->base, "absolute link is", $uri->abs;
[download]

This prints:

base is: http://domain.com/page
absolute link is: http://domain.com/page2
[download]

As far as I can say, WWW::Mechanize derives absolute URLs in quite the same way.