Should it be considered an error if syswrite writes some number of bytes different from the length of the string you give it?

No.

my $to_write = length($data);
my $offset   = 0;
while ($to_write > 0) {
   my $written = syswrite($fh, $data, $to_write, $offset);
   die("Unable to write to ...: $!\n")
      if not defined $written;
   $to_write -= $written;
   $offset   += $written;
}
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