threads

use threads;

async {
    `t1.pl "arg1" "arg2" "arg3"`;
}->detach;

print "DONE\n";
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Note: It doesn't make much sense to use backticks if you do nothing with the returned output; but that's not the subject of the question.

And having the first script end immediately after starting the second doesn't make much sense in isolation either -- why not just run the second script directly. But I assume this is just a generic example to satisfy the "where's your code" crowd.