It's not the debugger, I think the system command (aka backticks) is causing a flush

#!/usr/bin/perl
use strict;
use warnings;
use feature qw{ say };

unlink "./log.txt" or warn "$!";


open OUT, '>>', 'log.txt' or die $!;
print OUT "before system\n";

print `type log.txt`;

print OUT "after system\n";

open IN, '<', 'log.txt' or die $!;
my $nothing = <IN>;
say "Before close: $nothing.\n";


close OUT;
my $bla = <IN>;
say "After close: $bla.\n";
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Thu Feb 11 19:29:04

C:/Perl_524/bin\perl.exe -w d:/tmp/pm/auto_flush.pl 
before system
Before close: before system
.

After close: after system
.


Compilation finished at Thu Feb 11 19:29:05
[download]