With threads, just run each process in it's own thread and then use exit to terminate. Whichever finishes first will terminate the other thread automatically.

#! perl slw
#use 5.008008; For "no warnings 'threads'" ## Doesn't work?
use strict;
use threads;
no warnings 'threads';

async {
    no warnings 'threads';
    sleep rand(10); ## One longe running process
    print 'Thread 2 finished first';
    exit;
};

sleep rand(10); ## another long running process
print 'thread 1 finished first';
exit;
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The code will issue a warning A thread exited while 2 threads were running., but killing a thread this way is no worse that killing a process. From 5.8.8, no warnings 'threads'; is documented ... but not for this particular message it seems.