Just for completeness, here is an example using HTML::TreeBuilder:

use HTML::TreeBuilder;
my $tree = HTML::TreeBuilder->new_from_content($html);

foreach my $img ($tree->look_down('_tag', 'img')) {
    if ($img->attr('alt')) {
        print "Alt tag found: ", $img->attr('alt'), "\n";
    }
}

$tree->delete;
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gav^