What does the following error mean?

Use of uninitialized value at /usr/local/rich/spider.pl line 504.

I've cut line 503 and 504 and am pasting it here.

      for ( @$links ) {
            if ( $attr{ $_ } ) {  # ok tag


I'm using the -S prog method and SWISH-E 2.1-dev-25.

Thanks,

Rich