There is a nice chapter on just this topic in Perl Testing by Lanworth and chromatic (see http://shop.oreilly.com/product/9780596100926.do ) and gives clear examples using WWW::Mechanize. You'll be up and testing in no time.

To save you some time, here's a couple of hints from my brief experience:

# check the html on the page - skip this for brevity
$mech->page_links_ok();
html_tidy_ok( $mech->content() );
[download]

This does a lot of work in just two little lines.

$mech->content_contains( 'Sorry, we couldn’t find anything that 
+matched your search.' );
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This one took me forever to work out how to match the apostrophe. I went with the html entity in the end.