use strict;
use warnings;
use threads;
use threads::shared;

# enable autoflush on the handle
STDERR->autoflush;

my $sem_stderr :shared;
sub print_err { lock $sem_stderr; print STDERR @_; }

sub task {
    my $id = shift;
    my $str = "ab " x 3000;
    for (1..10000) {
        print_err($str, "\n");
    }
}

threads->create('task', $_) for 1..40;
$_->join for threads->list;