Simples:

#!perl -slw
use strict;
use threads;

sub one {
    select('','','',rand(0.01)), print 'one' while 1;
}

sub two {
    select('','','',rand(0.01)), print 'two' while 2;
}

async \&one;
two;
[download]

Doing something useful with it requires a little more thought.