ds: tmpio: avoid Perl target cache

The use of `substr' here an argument to `print' was causing Perl
to internally cache its target buffer.  Since `syswrite()'
already offers a buffer offset arg and length limits, just use
`syswrite' directly.  We were using autoflush anyways, so the
lack of buffering was of no concern performance-wise.

The target buffer could get to roughly ~10MB under some loads,
but it was usually a cold path and using memory which cannot be
released nor reused in other places.

note: IO::Handle::write uses `substr' internally, too;
so nothing would be gained using IO::Handle:write.

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