It seems GRAMPROG is not the only one that's special, I tried to search other symbols like ADDOP in toke.c
but it's never defined to "+" there.
where are these terminal symbols finally defined?
I think you're misunderstanding how this works. ADDOP (and many of the others) don't correspond to actual terminal symbols like '+'. They are just numerical constants (ADDOP is defined as 305 in perly.h). yylex returns this value for multiple "addish" operator types, such as '+', '-', '.'; with extra info in the ival field indicating what op to use (such as OP_ADD, OP_CONCAT etc).