What do you see if you run this?

#! perl -slw
use strict;
use threads;
use threads::shared;
use Time::HiRes qw[ time ];

my $count :shared = 0;

async{
    my $last = $count;
    while( sleep 1 ) {
        my $now = $count;
        printf "%d threads/second\n", $now - $last;
        $last = $now;
    }
}->detach;

threads->create(
    sub { lock $count; ++$count; }
)->join while 1;
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