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my $written = syswrite($test_file, $buf, length($buf));
die("Unable to write to file: $!\n")
   if not defined $written;
die("Unable to write whole string in one call\n")
   if $written != length($buf);

my $offset   = 0;
my $to_write = length($buf);
...
   $to_write -= $written;
   $offset   += $written;
}