remove redundant calls in S_minus_v in perl.c

remove redundant calls in S_minus_v in perl.c

Commit b0e47665895 added a large amount of redundant calls to
PerlIO_stdout(). Fix, cache stdout. Also commit 46807d8e809 added multiple
calls to SvPVs and sv_len on the same 2 scalars that do not change between
the calls to SvPV and sv_len. Since sv_len is a wrapper around SvPV*, just
use SvPV once on each scalar and cache everything.