A slightly easier to follow version:

use HTML::TreeBuilder;
foreach (<*.html>) {
    my $tree = HTML::TreeBuilder->new_from_file($_);
    if ($tree->look_down('_tag', 'img', sub { !$_[0]->attr('alt') })) 
+{
        print "$_ has a missing alt tag\n";
    }
    $tree->delete;
}
[download]

gav^