ExtUtils::Typemaps::STL::String versions before 1.06 for Perl T_STD_STRING typemap may read the SV length before stringifying the argument. The typemap uses $var = std::string( SvPV_nolen($arg), SvCUR($arg) ) However, evaluation order for C++ arguments is not specified, and some compilers may produce code that evalutes SvCUR($arg) first. When $arg is not a string (for example, an interger, number or a reference) then SvCUR will return an invalid value, and the program may abort or segfault.