Project:
toke.c
----------------------------
#include "EXTERN.h"
#include "perl.h"
void
S_force_word(register char *start, int token, int check_keyword, int allow_pack, int allow_initial_tick)
{
    PL_nextval[0].opval;
}
---------------------------

EXTERN.h
---------------------------
#define EXT extern
---------------------------

perl.h
---------------------------
#include "perly.h"
#include "embedvar.h"
#define PERLVARA(var,n,type) EXT type PL_##var[n];
#include "intrpvar.h"
---------------------------

perly.h
---------------------------
typedef union {
    int opval;
} YYSTYPE;
---------------------------

embedvar.h
---------------------------
#define PL_Inextval		PL_nextval
---------------------------

intrpvar.h
---------------------------
PERLVARA(Inextval,5,	YYSTYPE)	/* value of next token, if any */
---------------------------

a2p.c
---------------------------
typedef int YYSTYPE;
---------------------------

Add empty string to beginning of EXTERN.h and save
Add empty string to beginning of a2p.c and save
Add empty string to beginning of embedvar.h and save
Add empty string to beginning of intrpvar.h and save
Add empty string to toke.c after #include "perl.h" and save

Then there should be unresolved opval.