By using syswrite to populate env->{psgi.input}. The substr()
call IO::Handle->write will trigger Perl's target/scratchpad and
result in a permanent allocation. Since this is a cold path,
that allocation is pointless, and syswrite() can already write a
substring.
Allowing Perl to cache a large allocation in a cold path only
result in fragmentation and wasted RAM.
write(2) on a regular file won't result in short writes
unless the FS quotas or free space limits are hit, or the buffer
is close to overflowing (e.g. the 0x7ffff000-byte Linux limit).
Since our HTTP server will never buffer that much in RAM,
there's no need to retry syswrite nor rely on the retrying
implicit in IO::Handle->write and the "print" perlop.