Somewhat easier than using lock() all over your code, is to wrap print in your own sub that does that for you:

use threads;
use threads::shared;

my $sem :shared;
sub tprint {
    my $tid = threads->tid;
    lock $sem;
    print "$tid: ",@_, "\n";
}

sub twarn {
    my $tid = threads->tid;
    lock $sem;
    warn "$tid: ", @_;
}
[download]