Got it from HaukeX' recent post.

I should have read the FAQ: https://perldoc.perl.org/perlfaq5#How-do-I-flush/unbuffer-an-output-filehandle?-Why-must-I-do-this? more carefully :)

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

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



open OUT, '>>', 'log.txt' or die $!;



if (0){                                 # as you like
    my $previous_default = select(OUT); # save previous default
    $|++;                               # autoflush OUT
    select($previous_default);          # restore previous default
} else {
    #use PerlIO;                         # needed for Perl <5.14
    OUT->autoflush();
}


print OUT "before close";

open IN, '<', 'log.txt' or die $!;

my $read;

$read = <IN>;
chomp $read;
say "Before close: <$read>";

close OUT;
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Fri Feb 12 02:23:56

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

Compilation finished at Fri Feb 12 02:23:56
[download]