use strict;
use warnings;
use Fcntl 'SEEK_SET';

system "touch myfile";

open my $fh, '+<', 'myfile';

syswrite $fh, 'A' x 100000, 100000;

sysseek $fh, 0, SEEK_SET;

syswrite $fh, 'B' x 10, 10, 0;

close $fh;