avoid strange FreeBSD problem

avoid strange FreeBSD problem

<mxey> I have a weird problem with Ikiwiki on FreeBSD: When rendering this page <http://rafb.net/p/zmfROX86.html>, I get "ikiwiki.setup: Modification of a read-only value attempted at /usr/local/lib/perl5/site_perl/5.8.8/IkiWiki.pm line 1300.".

And converting the constructors to not operate on the parameters directly
but use lexical variables fixed it. Not sure exactly when this occurs.