The panic message originates in the module C::Scan, which is being used under the hood when you specify the -x option to h2xs.

The module has a number of known "issues" (and is unmaintained — last release 2001), which is why ModPerl::CScan was forked for use within mod_perl.  You could try to install ModPerl::CScan separately and replace all occurrences of "C::Scan" in (a copy of!) h2xs with "ModPerl::CScan", and see if it helps...