It might be worth adding a test to see if your URI object is actually being created.

...
       my $url = URI->new_abs($attrs{$a}, $base)
           or warn "Couldn't create URI for $attrs{$s}:$base : $! ", $
+^E||'';
...
[download]

I would have expected that you would be seeing other errors before this if the URI wasn't being created, but maybe it will show something that will help clarify the problem.