pp.c:pp_abs: No need for PUTBACK

We don’t manipulate the stack pointer at all here.
pp.c:pp_oct: Less stack fiddling

No need for pop and push, since we accept and return one item.  No
need to extend the stack.  No need for PUTBACK.
pp.c:pp_substr: Redundant PUTBACK/SPAGAIN

These were added in perl-5.004_03-1149-g849ca7e.  Perhaps at the time
SvPV or sv_magic could reallocate the stack.  Neither is true now.
pp.c:pp_ord: Less stack fiddling

No need for pop and push, since we accept and return one item.  No
need to extend the stack.  No need for PUTBACK.
pp.c:pp_chr: Less stack fiddling

No need for pop and push, since we accept and return one item.  No
need to extend the stack.  No need for PUTBACK.
pp.c:pp_[ul]c(first): No need for PUTBACK

We don’t manipulate the stack pointer at all here.