Hm ... I already mentioned $|

https://perldoc.perl.org/perlvar#$OUTPUT_AUTOFLUSH

Though the documentation says

> forces a flush right away and after every write or print on the currently selected output channel.

so, this means one has to select LOG before using it???°

IO::Handle mentions a method autoflush

BUT from my experience are opened handles per default unbuffered???

Or are they only line-buffered, and the OP forgot the trailing "\n"?

°) and after some digging I found this in the FAQ

{
    my $previous_default = select(STDOUT);  # save previous default
    $|++;                                   # autoflush STDOUT
    select(STDERR);
    $|++;                                   # autoflush STDERR, to be 
+sure
    select($previous_default);              # restore previous default
}
[download]

> Or are they only line-buffered, and the OP forgot the trailing "\n"?

Nope, from testing in the debugger, do new handles automatically autoflush

  DB<280> open LOG, ">>", "log.txt"

  DB<281> ! type log.txt
  DB<282> print LOG "bla"

  DB<283> ! type log.txt
bla  DB<284>

  DB<284> print LOG "\n"

  DB<285> ! type log.txt
bla
  DB<286>
[download]

probably is Catalyst bending the defaults?

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

Not sure about the debugger, but code shows otherwise:

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

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

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

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

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]