It seems that SvIS_FREED was introduced in 2005. Perl 5.8.5 was released in 2004, so it is unlikely that the module will work unmodified on your version of Perl.

You can try to paste the definition of SvIS_FREED into your code in the hope of making this module compile, but I recommend switching to a more recent and supported version of Perl.

Looking at the offending code, it only seems to be an assert() clause, so maybe you can just comment it out in the hope that everything goes well.