printf type-matching.
hv_store_ent() return value unused.

mg.c: In function 'Perl_magic_setlvref':
mg.c:2549:2: warning: value computed is not used [-Wunused-value]
pp.c: In function 'Perl_pp_refassign':
pp.c:6263:2: warning: value computed is not used [-Wunused-value]
HP-UX dynaloading uses shl_t, not void ptr.
switch-local variables are a bad idea.

(another weirdness is that the mg and stash are not explicitly
used in the code, so the macros must be assuming their existence.)

(clang -Wunreachable-code)
Hoist switch-local variables.

Additionally, svp was shadowed.

(clang -Wunreachable-code)