$ grep Perl_warner op.c

./op.c: Perl_warner(aTHX_ WARN_SYNTAX, "Found = in conditional, should be ==");
./op.c: Perl_warner(aTHX_ WARN_VOID, "Useless use of %s in void context", useless);
./op.c: Perl_warner(aTHX_ WARN_BAREWORD, "Bareword found in conditional");
./op.c: Perl_warner(aTHX_ WARN_VOID, "Too late to run CHECK block");
./op.c: Perl_warner(aTHX_ WARN_VOID, "Too late to run INIT block");
./op.c: Perl_warner(aTHX_ WARN_REDEFINE, "Subroutine %s redefined",name);
./op.c: Perl_warner(aTHX_ WARN_VOID, "Too late to run CHECK block");
./op.c: Perl_warner(aTHX_ WARN_VOID, "Too late to run INIT block");