If you’re a Perl developer on Mac OS X Leopard, then you’ve probably heard about this issue caused by Security Update 2009-001:

...the latest Mac OS X Security Update 2009-001 apparently breaks your Perl, if you

• are using Mac OS X 10.5 (Leopard)
• and are using Perl that comes with Mac OS X
• and have upgraded several core modules with CPAN in the past

I read about it a while ago but when I tried it on my machine at the time, I didn’t see the symptoms described in the article, even though I use CPAN a lot, so I thought that my system wasn’t affected.

Today, however, I had to track down a strange, hard to find bug where I would get messages like this:

Modification of a read-only value attempted

At first I thought I made some Perl beginner mistake, but in the end it turned out that I did indeed have a broken List::Util module with a version mismatch between the XS binary and the Perl module code. After downloading, compiling and installing a fresh copy of Scalar-List-Utils from CPAN, everything was back to normal.

Putting it here in the hope that it saves some other googling Perl developer some time :-)