I knocked up this random meaningless benchmark script:

my $t;
for my $i (1..10_000_000) {
    if ($i % 3) {
        $t += $i;
    }
}
[download]

and ran it a few times on a threaded and non-threaded perl build (a recent bleedperl) and got average timings of 4.46s and 4.99s, so about 10% slower.

Dave.