So, what's the best way to check output? Do you decided absolutely what the output will look like at the beginning of the project and hardcode it into a test? Do you just ignore testing output, and look at the output after every change? Do you do something completely different?

Some suggestions:

• Put as little presentational markup in the HTML as possible, relying on CSS for look and feel. This makes the underlying HTML far less likely to change.
• Test for well formed HTML, since that catches lots of typos.
• To get something quick and dirty up and running use a regex test, e.g. like( $obj->output, qr/relevant info/i ).

In general I try and make output layers as thin as possible, do as little as possible, and have as little knowledge about what they're presenting as possible. For example rather than having:

my $o = FribbleList::HTML->new;
$o->output;
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I'd tend to have something like

my $html_list_output = ListOutput::HTML->new;
my $o = FribbleList->new( output => $html_list_output );
$o->output; # really does $html_list_output->output( $o->list );
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so I can test my FribbleList class completely independently of my ListOutput::HTML class.