First, the comments in this block say one thing, but the code says something different

    # Set up a callback that collect image links
    my @imgs = ();
    my $callback = sub {
    my($tag, %attr) = @_;
        return if $tag ne 'a';  # we only look closer at <img ...>
        push(@imgs, values %attr);
    };
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You probably want to change this to:

    return if $tag ne 'img';
    push(@imgs, $attr{'src'});
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or

    return if $tag ne 'a';
    push(@imgs, $attr{'href'});
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Otherwise, if there are other attributes in the tag (alt=, border=) then you may end up getting their values instead.

Also, the abs() method to URI::URL returns a URI::URL object, so you'll need to change this line:

    @imgs = map { $_ = url($_, $base)->abs; } @imgs;
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to this:

    @imgs = map { $_ = url($_, $base)->abs->as_string; } @imgs;
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Which will give you a string, that can be used instead. For some reason this only fails occasionally, so this may be the problem that you are running into.