I could not recreate the problem following your instructions above, but had the same issue a couple of times lately. Unfortunatly I cannot recall what I did before it happened (same browser).

UPDATE: The problem seems to come from a slash in the url: somehow (right after posting) it became http://www.perlmonks.org/index.pl/?node_id=400961 and the relative link to index.pl would therefore take me to /index.pl/index.pl. IE is doing the same. Maybe a link to "/index.pl" would fix this?