#! perl -slw
use strict;
use threads;

#...

my $t1 = async{ `perl -e"print, sleep 1 for 1 ..10"` };
my $t2 = async{ `perl -le"print, sleep 1 for 'a'..'z'"` };

#...

my $output1 = $t1->join;
my $output2 = $t2->join;

print for $output1, $output2;